Most machines now configured with 4GB of RAM but continue to be bundled with the 32-bit version of Windows. You might’ve noticed that in this situation it does not fully utilise all of it, but here’s a simple fix to work around this issue instead of putting a 64-bit OS instead:

To enable PAE mode, you have to add PAE to the boot entry in the BCD file. Open an elevated command prompt. Type BCDEDIT /SET PAE ForceEnable.

via: Microsoft’s help article