Physical memory increase tool in xp

And???
Add a new RAM Module to the existing lot, if you have any RAM slots on your mobo which are left unused.

If it's a laptop, then get another SO-DIMM matching with the already existing SO-DIMM module in the laptop...

To check what RAM is being used without opening the case download CPU-Z from cpuid.com and check the Memory and SPD tabs....
That will tell you the make , model and timings of your existing RAM modules....
 
Then you're probably running either WinXP or WinVista 32bit operating system, you actually have 4G of physical hardware memory modules, but the operating system is reporting a lower amount, that's normal for a 32bit OP/SYS, a 64bit OP/SYS will show the full 4G of system memory.