In a nutshell, use a 64-bit operating system (and machine). Operating systems like Windows and Linux do not allow direct access to RAM (unless you are part of the kernel, or a device driver). Instead they use Virtual Memory.

The program needs to access the memory area using an address, and the largest that can be held in a 32-bit word is 4GB.
So where did the other GB go?
That is reserved for kernel.
Kernel uses a Gig?
No, that address range is reserved for kernel use, it doesn't use anything like that.

A 64-bit architecture will, in theory allow an address space of up to 16 Exabytes.