How to enable 4GB in my Windows 7 64 bit?

Solution 1:

Your BIOS needs to support memory hole remapping.

Memory hole is there since all devices must fit in 32-bit address space during boot (since BIOS is not aware whether you will use 32-bit or 64-bit OS later). Remapping takes care to move affected memory to memory area after 32-bit boundary.

I would check whether new BIOS firmware is available if that setting is not in your current BIOS firmware.

Solution 2:

Josip is correct that the BIOS with memory hole remapping is necessary. As Chris_K tries to say your hardware must also to be able to physically address more than 4GB see KB929605. I believe that the Intel 945 chipset family cannot do this. So, sorry to say you are out of luck.