You probably have a 32 bit version of Windows. A 32 bit CPU has a 4 GB address space. But Windows CPU's use part of that address space for something called "memory mapped i/o" (google the term). That reduces the amount of memory that the CPU sees. If the motherboard depends on chipset graphics, that further reduces the amount of memory that Windows sees.
Entering the BIOS will tell you what the system sees. That should be 4 GB .
32 bit windows supports 4GB max. You will see between 3-3.5GB. If you get a 64bit OS, it will give a full 4GB.
I installed Windows 7 Enterprise on the same model laptop (a135-24487) and it was telling me the same thing, only 3gb usable of the 4gb memory installed. I waited specifically till I had a 64-bit version of Windows before upgrading the ram to be sure it would be worth if by having my laptop use all the ram possible.
Could there be a bios setting that needs to be changed to have it use all 4gb of ram? It sees all 4, I just can't figure out why it wouldn't use all 4?
If you have 64bit installed, than it is a bios issue. If youre laptop was intended for 32bit and they haven't released a Bios update, than youre out of luck. I had a similar problem with an Asus board of mine. I installed 8gb and it showed 3.99 usable. They came out with a Bios Update to fix the memory problem thankfully. But yea you need to check if your bios is up to date, and if there is a newer version, flash it. If there is not, youre out of luck.