Did you enable memory remapping in the BIOS?
The BIOS should detect the 4 gig, once you boot into Windows however, that is a different story. A 32 bit OS will not be able to use all 4 gig of your main memory. It can address 4 gig, but that is the total memory counting ALL the memory in the system (video cards, hard drive cache, processor cache, etc.) It will address these items space first, and whatever is left over will be addressed for main memory. Depending on what else you have in the system, most people end up with around 3 gig of useable memory.