32-bit x86 RAM limit
In non-PAE modes of x86 processors the RAM memory is always limited to 4GB.
Limits on memory and address space vary by platform, operating system, and on Windows by whether the IMAGEFILELARGEADDRESSAWARE value of the LOADED_IMAGE structure and 4-gigabyte tuning (4GT) are in use.
Limits on physical memory for 32-bit platforms also depend on the Physical Address Extension (PAE), which allows 32-bit systems to use more than 4 GB of physical memory.
PAE and 64-bit systems can address up to the full address space of the x86 processor.
I would first of all try reseating the RAM or one stick at a time.