This is a combination of info from a dozen places, some of which I had to draw from memory. There could be inaccuracies.
Back in 98-99, MP3 was the biggest buzzword around. A few years later, when the engine that we would later refer to with metanyms like "aliexpress" got into full swing, incredibly cheap and miserable portable audio players began flooding the market
These things were awful. I doubt the ones I just screenshotted from google shopping are any better, because it was extremely obvious at the time that they were all derived from the same SoC loaded with the same software, for years and years and years, and that software was garbage. As in, "SimSun font with subpixel AA turned on even though the screen is so low-res that the text just turns into a rainbow." Button lag, indecipherable display, the works.
The handle we used at the time however was "MP4 player," because that's what they all said on the box, to a one. They all seemed very proud of it, despite it being completely meaningless nonsense as far as anyone I knew could tell.
It seemed like a "Nintendo 65" kind of thing, like the people who had hastily and unthinkingly labeled these had no understanding of the technology and just went "oh, everything does MP3. let's say it's MP4. that's one better." This actually happens, a lot, if you flip through Amazon listings. There's some wild shit in there from people who clearly did not know anything about what they were being asked to write copy for.
I didn't think about this for decades, other than to periodically go "oh lol mp4 player. that sure was a time. lmao." But over the last year I obtained some new information, and the pieces finally slid together and clicked about ten minutes ago.
Not only was "MP4 player" accurate, you probably owned and used one heavily.
(you can scroll to the last section if you just want the quick answer.)
The development of MPEG is harder to follow than the release history of the Wonder Boy series. The people who designed the standards made some very interesting decisions around versioning and terminology, which didn't matter much at the outset because most nontechnical people didn't even know those standards existed.
Then MP3 entered the public consciousness - which automatically renders any technical concept about 50% less coherent as it gets telephone-gamed around at blistering speed - and if that wasn't enough, it was also subjected to a weird branding / decoupling effort at the last practical second which made things even more confusing.
Most of us probably know that MP3 is not "MPEG3," but rather "MPEG Layer 3." The actual terminology however is even more confusing.
Each MPEG standard has parts, which are things like "container", "video" and "audio"; none directly define a format, just high level concepts and framing. Within each part, there are multiple options for what actual codec or data format to use - but they changed the terminology for this between variants.
In MPEG Part 2 (Video), there was only one codec available, very similar to the extant H.261* (though technically distinct.) But Part 3 (Audio) supported three different codecs, called layers.
I'm not sure if H.261 is the refined and internally-standardized form of MPEG video, or if MPEG was "based on" H.261; I will ignore this for the rest of this longpost to save on space, but keep in mind that either way is possible.
MPEG Part 3, Layer 1 and Layer 2 were variations on an audio codec that was in turn based on 1989's MUSICAM. If I open up any random .MPG file from the 90s, that's probably what I'll find. My Hitachi MP-EG1, for instance, uses Layer 1 (the lower bitrate version that's easier to process in realtime on crackerjack SoCs.) The Beavis and Butthead VHS rips I downloaded from Internet Archive use Layer 2.
MPEG Part 3, Layer 3 was the Fraunhofer codec we know and often detest, and one of my favorite Computer Facts is that it was present in the original 1993 MPEG standard - though, as far as I can tell, it wasn't actually implemented yet. It was very computationally expensive, and ostensibly Fraunhofer didn't even have a reference implementation until 1994, but it was technically a valid codec option, two years before Windows 95 existed.
Part 1 of MPEG describes a container format which encapsulates these other Parts. It has a video branch and an audio branch, which makes it unfit for carrying just one or the other, so when you have a file that only contains one Part, it doesn't use the complete MPEG format. I believe a file containing just one part is called an Elementary Stream, but don't quote me on that Part (ha.)
MP3 is one such Elementary format. It's just Part 3, Layer 3, distributed by itself without the full container. Intriguingly, the Fraunhofer Society initially planned to make the canonical filename .bit. I don't know why they didn't. This could have happened. We could have had BIT players instead of MP3 players. I'm not sure whether that's better or worse, but at any rate, they changed it to .mp3 shortly before publishing the first encoder.
Even more fascinating: Wikipedia says Layers 2 and 3 can be distributed on their own as .mp1 and .mp2 files. This is kind of terrifying and I'm glad I've never seen it.
I always felt that there was a perception that MP3 was short for "MPEG3," though I can't prove it. It's the sort of thing that I think nerds liked to sit around and imagine "lusers" did, even if we had no proof of it. And I'm sure the academics who designed the format would have liked to call it "MP3L3" for clarity, but since FAT only permitted 3 letter extensions, this is what we got.
If this confusion happened though, it's partly attributable to the weird seesawing timeline of these formats. Roughly, it went like this:
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1993: MPEG (retroactively, MPEG-1) is initially released. Technical people become familiar with it fairly quickly.
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1994: MPEG-2 is released, but not used for much yet
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1995: Windows 95 is released, which includes MPEG files on the CD version. Many consumers become passingly familiar with MPEG.
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1996: DVD enters the market, using MPEG-2 compression.
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1997: The distribution of MP3 files begins in earnest thanks to Winamp and Napster.
MP3 predates MPEG-2, but the greater public didn't know about either of those until after MP3 became well known. Many people who used MP3s had probably never played an MPEG file, and given how confusing Online was in those days (not to mention people's butchered explanations of things around the water cooler) anyone who began to dig into the format was very likely to learn about the existence of MPEG2, and assume that MP3 - which had only become publicly relevant after DVD, the most well known use of MPEG2 - was a newer format.
To confuse matters more: there was an MPEG3 project (intended to support HDTV) but it was cancelled... in 1992. Before MPEG2 was even finalized. But it gets weirder even from there.
MPEG-2 Part 3 had the same options as MPEG-1 Part 3. Layers I, II and III were the same audio codecs, but with different bitrate options and multichannel support.
However, the standards writers later added a new audio codec, that wasn't present in MPEG-1. Was this "Layer 4"? No - Part 3 was already finalized, so instead it was MPEG-2 Part 7.
That's right: The video stream on a DVD is MPEG-2 Part 2, but the audio stream is either Part 3, Layer I-III, or Part 7, Layer Nothing.
I can't tell if this was retroactive or not, but internally Part 3 is called MPEG-2 BC (Backwards Compatible), and Part 7 is MPEG-2 NBC (Non-Backwards Compatible.) Hilarious, especially since this new codec was Advanced Audio Coding, which we know as AAC, but easily could have become known as NBC.
This, of course, brings us to the MPEG4 mess.
I talked about MPEG4 in a video a while back and frankly I think I covered it most concisely there.
MPEG4 was much like its predecessors, with a standard video codec and a number of options for audio. Then, Things Happened.
The MPEG4 container format, for whatever reason, became extremely popular for encapsulating other codecs. I don't know why this didn't happen with MPEGs 1 or 2, since it had been going on with .AVI for some time. MOV got the same treatment, and it led to the FourCC Hell we lived in for about 15 years, where you couldn't tell if any given file - be it .mp4, .mov, .mkv or .avi - was playable until you tried to play it.
That nightmare largely ended a few years ago, initially because VLC put non-gunky versions of every imaginable codec on everyone's desktop, but mostly because virtually everything rapidly standardized onto H.264 and H.265, which are included with all modern OSes. Thank christ, because Codec Packs were hell on earth.
This, however, is a lot of why we made fun of "MP4 player." MP4 what? You're gonna tell me these fake iPods with 162x84 LCDs can play videos? And any of the dozens of codecs that could be in there? Gimme a break.
But what I didn't realize until I was researching that video last year is that MPEG4 itself actually did specify standard video and audio codecs, and for several years, they actually meant something.
Initially (just like MPEGs 1 and 2,) MPEG4 had a Part 2 that described a single specific video codec. It was largely based on H.263, just as MPEG-1 had been .261 and MPEG-2 had been .262 - but for some reason nobody started using those terms until H.264 came out. It's entirely possible that these terms didn't exist until years later, I have not dug that deep.
At any rate, there was no specific way to refer to MPEG-4's video except as "MPEG-4 video", and that's exactly what people had done in the past with prior standards, but weird things started happening at this point.
MPEG-1 was mostly integrated into programs, or came on CDs that included the codec and told you to use Windows Media Player. MPEG-2 was virtually unused by consumers outside of DVD, so almost nobody really saw it directly. These formats were not tremendously hands-on for most people, and there was little reason to refer to them.
MPEG-4, on the other hand, entered a world that was primed for using video On The Computer, so for the first time, millions of people (instead of a couple hundred thousand nerds) wanted to fully utilize it; to play it, send it, author it, and discuss it. It became important for websites to be able to describe what formats a video was available in, so terminology was needed, but there was no simple terminology.
Unlike MPEG-1, there were no free (or generally available) implementations of the MPEG-4 video codec for years after it came out. Not that there was ever an "official" implementation; all video codecs up until that point (and probably still) were simply descriptions of a file, and the actual software was always created by private businesses. This happened with MPEG4 as well, but none of the companies that implemented it called it MPEG4.
The first MPEG4 file you ever played was probably a tossup between:
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WMV, aka WMV7, aka Windows Media Video, which was actually an incompatible implementation of a specific variant of MPEG4 Part 2 called Advanced Simple Profile, or ASP. Because MPEG4 Part 2 had Layers, but they didn't call them that. For some reason.
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DivX 3, a stolen, cracked version of Microsoft's earlier MPEG4 implementation, which they called MPEG-4 Version 3, which was also incompatible both with MPEG4 ASP and with WMV7
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DivX 4, a commercial cleanroom reimplementation of MPEG-4, also incompatible with everything else, but capable of playing DivX 3
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Xvid, a free codec created by open sourcing part of DivX 4.
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Any of about three other codecs that were illicit adaptations of Xvid. One was called AngelPotion and you can read a beautiful treatise on it from the Virtualdub guy.
All of these are, and aren't, MPEG4. This is the real hell of the FourCC Wars: Throughout all that absurd, clown college bullshit you had to go through, you were fighting to get FOUR DIFFERENT COMPETING IMPLEMENTATIONS OF THE SAME FUCKING CODEC to coexist.
These were almost never inside an MPEG4 container, by the way. I don't know why. Microsoft invented the ASF container for WMV (the filenames .ASF, .WMV and .WMA described the exact same file format.) DivX and XviD usually ended up in .AVI.
Later on, for some reason, the MPEG4 container became popular and is still used to date. Most of the time, it contains H.264 or H.265 - and in fact, H.264 was later bolted on to MPEG4 as Part 11, so it's all good, right?
Well, no, because H.265 never was. That's MPEG-H Part 2.
The 2000s were hell on earth, so needless to say the audio portion of MPEG4 is no cakewalk either. This is where we get back on track with the original point of this post.
MPEG-4 Part 3 handles audio, but doesn't have layers either; they got rid of that concept after MPEG-2, for some reason.
Instead, it has a bunch of Subparts, which collectively describe 46 separate "object types", many of which are codecs. There are then "profiles" which mix and match those codecs. I won't even begin to try to understand any of that.
I don't think any of it mattered, either, because in practice what I believe occurred is that everyone just used MP3 or AAC. The files from my Sharp VN-EZ1, the first consumer device to use MPEG4 (afaik,) use AAC, so I suspect nobody ever bothered with any of that other nonsense.
It would probably be safe to say that MPEG4 Part 3 is AAC. And the funny thing is that AAC is specifically in Subpart 4, which might as well be Layer 4. And that means that, despite all this messy shit, all the ups and downs and confusion and switchbacks, it would be perfectly accurate to say that AAC, in a file by itself, is .mp4.
Except that's not what we did. We just called it AAC. Which means that the iPod, for many people, was only an MP3 player for about two years, and when the iTunes store came out and everyone began buying music that way, the iPod primarily served as an MP4 player.
Presumably those no-name Chinese devices could play these as well. We had been dicks all along.
