Hi.. Can you check the following:
1. What does the BIOS say about your memory? If BIOS Says, 4GB, then nothing much we can do... That means Your Chipset supports upto 4GB.(You are using 64 bit OS doesnt make all 8GB available)
2. Which version of Widows 7 you are using? The memory that is usable is depenedent on the Chipset also. Please visit:Memory Limits for Windows Releases (Windows)
Windows 7 Part for more details
3. As you are using 2 4GB Rams, it is also possible to be a loose connection for the second RAM.
Let me do more research on this for you.... You Plz check these first, lets see what best we can do...