I think you can go to 8GB
http://www.crucial.com/store/listpar...567%29&Cat=RAM
But I'd need the full model number to be sure.
If you are buying RAM, I can't recommend buying Crucial RAM enough, it is a little bit more, but I spend a huge portion of my life troubleshooting laptops, and have lost count of the amount of times replacing the RAM has solved their problems. Many respected brands I have had endless trouble with, I have never, and I do mean never had a single problem with Crucial RAM, in literally thousands of machines over my career.
Oh, and here is how to restore your machine to its original state.
http://aps2.toshiba-tro.de/kb0/HTD1303440001R01.htm