But I should add to that that I have the dual extruder running perfectly well with an old RAMBo (with the 8 microstep motor drivers). It's hard to guess at what combination of firmware and hardware causes the problem, but it is possible to run old electronics with the new v4 and dual extruder hardware.
On the other hand, I'm an Electrical Engineer, and the electronics and firmware are the part of the machine that I'm most comfortable with.