I recently went thru a similar decision process.
I have a case that I like very much. It stays cool and is QUIET. But it's in the BTX format, which altho I like very much, apparently the world didn't like enough to really catch on.
So to keep my current case, I had to choose from kinda older mobos, and can't fit all the best cpus.
In the case described here, I don't quite get why the OP wants to stick with Pentium 4. If you upgrade both mobo and cpu together, there'll be fewer constraints on what will fit.
But in general, I think it's often just fine to go with older/weaker technology. For most users, it really doesn't matter to get the fastest system possible, if the main tasks the computer will do are web-surfing, office tasks, etc.