How to Check RAM Size without Looking at Motherboard

It's because your OS is 32-bit. Check out Dude, Where's My 4 Gigabytes of RAM?

http://www.cpuid.com/cpuz.php

This will tell you everything about the memory installed in your machine along with the cpu. It will show which slots contain what size sticks and what speed they are running at.

The link JP posted from coding horror explains exactly why you only see 3.5gb out of the 4 you have installed is because of a 32bit OS.

Because you are running 32Bit Windows 3.5GB is the maximum amount of RAM you can support.

A 64bit Operating system is required to use any more RAM. So currently you are wasting 512MB of RAM.