I tend to use my right side over my left, and the top pair of arms over the bottom, so out of the four I would say the topmost right one is my "dominant hand".
I initially misread this as asking how typing work with four hands, so I'll add that on as well. If I'm just typing, I would default to the top two, and potentially use a third for arrow keys or the numpad (although my keyboards are currently only 1 for 3 on numpads). If I'm using a mouse at the same time, it's sometimes easier to type with the bottom hands, depending on the desk height, and use the mouse with my dominant hand instead.
