I'm excited that you're interested!
Probably the most confusing part of the pony numerical system for people who already understand this type of math is actually the linguistics of it: the ponies are referring to a base-4 counting system as "decimal". Obviously, the English word "decimal" means base-10, but I wanted to explore the intersection of linguistics with "hard" sciences like mathematics-- in this case, I discarded the literal definition of "decimal" as base-10 and instead interpreted it as "the root base that makes the most intuitive sense". With that interpretation, human decimal is base-10 because our number system is based on counting on ten fingers, and pony decimal is base-4 because it's based on counting on four hooves.
If it's the math itself confusing you, that's fair! Thinking in different base counting systems is really hard, and base-4 isn't as well documented as binary and hexadecimal, mostly because it doesn't have any computer science application. You can read about base-4 here: https://www.purplemath.com/modules/numbbase2.htm
When it comes to including numbers in other parts of the writing, it's pretty tricky because obviously English (and any other human language) is only designed for talking about numbers in base-10. As a general rule, if a number is written out in words, that's the intuitive base-10 number you're expecting, and if it's in numerals, that's either base-4 or hexadecimal depending on the target audience of the in-universe website. To make it a little easier, I include a hover tip with the normal human base-10 numerals on any number that's written in base-4 or hexadecimal.
I really hope that made sense and didn't just confuse you more! ^^; Thank you so much for asking because I LOVE talking about this stuff!
