While the previous responses are technically correct, this sounds odd to me. How exactly is your RAM configured? What system/motherboard and processor are you using? What DIMMs are you using, and what slots are the plugged into?
Most systems need to have the DIMMs installed in pairs across channels or they act odd. While Intel chipsets and processors technically can handle differing modules, they can act in unusual manners. I know of one circumstance where a guy had a 2GB module, a 1GB module, and two 512MB modules, and his system counted it up, but would only allow for 2GB to be usable. As soon as he changed out the modules to just two 2GB modules, he could access all the memory without issue. I'm thinking this might be a possibility in your case, but I need more information to determine this.