Why windows not showing 4GB ram?

There's a chip on your motherboard that is sometimes referred to as the "north bridge." One of the responsibilities this chip has is to translate information from your CPU to your RAM and vice-versa. But it can only speak a 32-bit dialect of RAM, which limits it to 2^32 bits of available addressing space. So no matter how much RAM you install, your total system memory is limited to 2^32 bits, which is about 4GB. Since other devices require addressing space as well, this traditionally lowers your RAM allocation down to somewhere in the 2.5 to 3.5 GB range, depending on your hardware.