Probably OK, but at a much higher price.
Most probably not. I would guess that some useful features are removed, or perhaps solved a different way so that users of Mach3 must modify their work flow or configurations.
What is the definition of "complete" and "stable"? Even Mach3 has it's weaker moments and every software needs an update now and then.
The way I see the biggest disadvantage of Mach4 vs. Mach3 is the price. It is in my opinion too expensive, so for me there is no interest in it. If I have to buy a new license I might as well try something else, like UCCNC.