Hell no. The G4 was good for its time, but now its just old, outdated and slow. I had a 3GHz P4, but my MacBook is faster. My MacBook pulls about 1.8Gigaflops in Geekbench 😀 .
Edit: Actually, for "power / square inch", I bet the Mini would win. I bet the Mini is the size of the heatsink of the P4 😛.