Probably no physical fit for a P4. You did not mention the MB but it will likely be a Slot 1, socket 370 or such (probably the later). P4s will be socket 478 meaning more pins. There are other reasons. Your chipset won't be right. Sorry it's not what you were hoping for.
I have a P3-866 and was wondering how i can find out if i can upgrade to a P4 chipset using my existing motherboard? and if i can what do i look for? if they (my existing & a new) have the same # of pins is that good enough? I also have (2)stick of 256 RAM and (1) stick of 128 RAM and my system tells me that it has 384 mb of RAM....Y? what am i doing wrong? (the (2) sticks are PNY and the 128 came w/ the sys.)

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic