it realy depends on the version of windows.
if you have a 32-bit version installed, then there is no reason to upgrade since any 32-bit windows version cannot use more than 3.5 GB of memory.
if you have a 64-bit version installed, then you might see a difference since it should be doing less swapping to the hard drive.
the other question is does it currently have one 4GB or two 2GB memories installed.
should it have two 2GB installed you will have to buy 8GB.