Mach 3 runs fine for me. Once I got it all setup and quit messing with it, the combo is very stable. Mach 4 has smoother motion and eventually will have more features. For instance, Mach 4 has a version that has Fanuc Custom Macro B where Mach 3 will never have that. But if you want a machine running now or yesterday, then Mach 3 is what I would use. Simply because the Galil plugin for Mach 4 is still in development.
Steve