Question : Problem: why when i put 4g ram on win xp appear 3g

my pc have two slots and i put 2 pcs of 2g ram total 4g ram in the bios it appears normally 4g but in win xp proffisional it appear and recognized as 3g just my mother board support 4g of ram and its bios is new and up to date

Answer : Problem: why when i put 4g ram on win xp appear 3g

That is normal, unfortunatly.  The x86 architecture can address just 4Gb of space, but maps IO, video etc into some of that space. This is subtracted for your 4Gb.  Typically, a PC with 4Gb of RAM running any 32Bit OS will see between 3Gb & 3.8 Gb.  You may be able to get a little more by updating or fiddling with BIOS settings, but you will never get the full 4Gb.