I have a zotac gt 430 4gb graphics card!

i have a zotac g 430 graphics card. I got it 2 days back! i installed it! i t actually has 4gb ! wen i checked using dxdiag it shows only 725 mb! and it keeps on reducing each time i check! now its 710! how to change it back into 4gb??
I cant change the graphics in gta4 due to that! i used to have an ati 4670 graphics card which was 3 times better than this 1 please help me!