As per Toshiba specs:

"Operating System:

• Genuine Windows Vista® Home Premium (SP1, 32-bit version)

Memory:

• Configured with 3072MB PC5300 DDR2 SDRAM (both memory slots may be occupied). Maximum capacity 4096MB"

- Technically:

I am not sure modules of 4GB DDR2 laptop RAM exist. I believe they are a maximum of 2GB RAM per stick. But don't quote me on that. So probably unfeasible to upgrade beyond 4GB RAM.

- In theory, you should have 2GB+1GB RAM already installed; if you decide to upgrade to 4GB (2*2GB RAM, by replacing the 1GB stick), consider the following:

-- Your system is 32-bit Windows XP, unless you already upgraded it. You will need to upgrade to Windows 7 64-bit to take advantage of the 4th GB of RAM and see some difference;

-- Your system is an 8-year old Pentium with 1MB Cache. Expanding the RAM by 30% may not add much. The question here is, what do you expect to achieve by adding the extra RAM?

-- Your 8-year old HDD probably is at its last breathe. Consider replacing it instead. Ask a separate question on whether your system can handle an SSD (I am not sure of the answer) which I believe will be a sounder investment and probably add more zest to your computer.

Conclusion, in my opinion, it does not make sense to use more than 4GB RAM with a 7 years-old Pentium even if you could (technically, it does not seem feasible). Practically, I believe it will not amount to much improvement, better increase the RAM to 4GB by adding a 2GB stick, upgrading to Windows 7 64-bit, and investing instead in an SSD which will improve your system; and you can take that SSD with you to the next system or use it as external storage later on when your computer dies or you move on.