The Windows 7 Operating System may not identify 4GB RAM and will only display 3GB or a little less, if the motherboard of your PC is very old and/or incompatible to run at 800 MHz speed. Go through your motherboard's manual by downloading it from the website to know about the configuration and compatibility issues. The memory remap feature must be turned on in the BIOS. Upgrade to a Windows 64 bit CPU to fully utilise the 4GB RAM. The issue may also be resolved by adjusting the memory settings in the BIOS and making it up to date.
Windows 7 Experience Index points: 4 GB (2.94 GB usable)
This happens with old motherboards, though Windows 7 detects the 4 GB RAM, it puts a limitation on its overall use.
This is normal if you are using a 32 bit version or the BIOS is incorrectly set (64 bits).
(Memory Hole remapping).