And then people ask these questions and we answer them like a actual helpful person (Which you are not) @friscowz, It's about being helpful, not being a idiot. Please stop posting useless comments like above. It's just making the forum messy.
Cuz it was one of the last common 32 bit CPUs and fits exactly within the time you mentioned Not only that, but it would probably be really slow as the Pentium 4's are all single core. And also, a Pentium 4 is power inefficient as hell, so K can't really recommend running that thing 24/7.
Well it wouldn‘t be slow because being single core (pmmp doesn‘t use much multicor, except for world gen), however it would be slow because of the low clockrate.
But the OS PocketMine is run on will need some power too, dual core would help that. btw. there are some Pentium 4's with a really, really high clock speed, but pretty uncommon.