I have always understood the minimum CPU requirement was a 233Mhz Pentium. I can't say first hand that it won't work, but if Windows XP is anything like Windows ME was (with regard to system requirements), if it detects an inadequate CPU it will not allow you to install. However I can not confirm this as I have never actually tried to install XP on a machine with specs that low.
Which brings me to my next point. Like
Prime' said, even if you could get it to install, it would be completely un-usable. Even if you upgraded the memory on that machine, it will still run so slow, you would have to wait half an hour just for the calculator to load up.
I personally wouldn't install XP on a system with less than a 500mhz chip. I installed it on a K6-2 550 (I know they aren't exactly great chips but nevertheless) a while back and even then it was only borderline usable, and that machine had 256MB of RAM. Windows 98 is really your only option.
Personally though I don't think it will even install, but that's just me. But that point is pretty much moot considering the fact that even if you get it installed, and your friend is willing to wait half an hour for apps to load, he probably won't have enough hard drive space left on that 4gig to run anything else anyway.
http://support.microsoft.com/default...b;en-us;314865