The difference between Piii & Piv would mean replacing not just motherboard but CPU, RAM, Powersupply and probably graphics card as well. You are looking at a completely different system. The only parts you could transfer from a Piii to a Piv would be the case and drives. Even the operating system would have to be re-installed. If you want a P4 system you can buy one for next to nothing.
Goin' Fishin' (Some day)