Do G4s have the interrupt switch? I believe some blue/white G3s do, it's one of the two little buttons below the power button. The one that sticks out is a reset switch and the one that sticks in a little bit is an interrupt switch, as far as I can recall.

I do not, however, recall what the interrupt switch on a G3 would do, compared to what it'd do on a 68k Mac that has it.