The reason why your computer is only reading 3.5GB of RAM is because the other .5GB is probably used for your videocard or for other resources on your computer.
• BIOS – including ACPI and legacy video support
• PCI bus including bridges etc.
• PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory
source:
http://blogs.msdn.com/hiltonl/archive/2007/04/13/the-3gb-not-4gb-ram-problem.aspx
You said that you also tried installing 4GB more RAM. So are you saying you tried installing 8GB of RAM? Are you installing the same type of RAM? Are the RAM made from the same company? Does your motherboard even allow more than 4GB of RAM? Some motherboards won't boot if you install 4 DIMMs of RAM.
What is the specs for your entire computer? motherboard, type of RAM, etc? I can't help you, if you don't give me more information.