Mach 3/4 will only work with Windows XP for starters.
Art has said that mach4 requires less performance out of the computer. Therefore you should be able to use it on a computer with less than the current minimum requirement of a 1 Gig processor fro mach 3.
Note: Mach 4 has not yet been released, and there is only a alpha version currently availiable. Alpha, not even Beta. So beware.