System core works with 32bit digit length, and 32bit = 2^32 = exactly 4Gb. So the maximum amount of MEMORY (i'm not saying ram) available for your system is 4Gb.
But, you must understand, that this maximum amount works this way:
Maximum available for RAM = Maximum total (4Gb) - Processor memory (±2 or 4Mb) - Video Card memory (how much do you have? 128? 256? 512?) - Reserved by the motherboard itself for any other controller.
So in total, the maximum usable memory capacity will be ±3.2Gb for RAM.