That would depend on a few factors.
If you are running 32 bit XP then you would see some system improvement dependant upon what you use your system for.
Some applications (some, but not games, video editing/encoding, graphics etc) would see better performance than others ( video watching, browsing), but in general terms a system would benefit from the extra RAM. Bear in mind that under a 32-bit, the operating system can only utilise (address) approximately 2.5-3.5Gb of the available 4Gb - basically 4Gb minus your video card memory and address space allocated to hardware resources. If you have a 64-bit version of XP then no such limitation applies, at least as 4Gb goes.
If 4Gb is not significantly more expensive than 2Gb then it's a viable upgrade (again dependant upon what app's you run) even though you may not utilize the whole 4Gb.