What speed is your G4. I've just got the MK games running fine with sdlmame 116u2 on my dual G5 2.0 with -mt on and some of the openGL enhancements that are in the new sdlmame. Prior to this even with as powerful a computer as mine the MK games would slow down with certain bulids of mame, if I remember correctly these games don't run as nicely on PPC chips as they do on X86. I'm sure someone else here has a more in depth explanation, but I've forgotten the exact reason to this, it's just a price us poor PPC users have to live with
off topic I've also noticed that on the MK games when you resume from a save state the audio sometimes goes to pot, it's out by over one second, easy to fix just test the sound board and it'll work fine again, but defeats the purpose of having a save state.