A 32 bit OS will not use more than 4 gb of ram inspite of adding more. This is due to many hardware architecture limitation
2^32 = 4294967296 bytes = 4194304 kb = 4096 mb = 4 gb
Install a 64 bit OS to use all of your RAM.
For viewing (not using) more than 4 gb of ram PAE (Physical address extension) must be enabled.
However there exist certain hacks such as this one which which make use of all available ram however this heavily modifies your kernel, and your system might become unstable.
Note: this hack is only for Windows 7 32 bit
Until you can upgrade to a 64 bit OS you can use this software to make use of the additional unused ram as a high speed disk drive (cache)
A similar freeware version of the above software exist with limitation of only be able to support disks of size upto 4 GB. Check it here, check this too, or Google for more alternatives, if you are interested