1. Try opening task manger and seeing how much RAM the operating system is actually using, because the RAM testing program you used might be faulty.
2. Do some research to see if your motherboard actually supports 4GB of RAM, because it might not.
3. If none of the above works, then start up in BIOS, and see if there is a setting in there that is preventing you from using over 3GB of RAM.
4. If still nothing works, then take out each stick of RAM and pug it into another machine to test if they are faulty, or if you don't have another machine, then pull all of the RAM out, except one, and boot up. Try this with every stick of RAM, you will know if one is faulty, due to the beeping noise at start up.