Sony Vaio RAM Upgrade

You'll have to find out what type of RAM your laptop supports before trying to install another stick of RAM. If you download CPU-Z and click on the Memory tab it'll tell you the type of RAM your laptop is using, its DRAM Frequency (You'll have to multiply it by 2 to get the actually frequency as DDR stands for Dual Data Rate) and the RAM timings.

The bit you need to be concerned with is the type of RAM (e.g. DDR2, DDR3, etc). You'll have to get the supported type. If you buy a 4GB stick of the same type with the same Timings and DRAM Frequency you should be fine. If the frequency and/or timings are different, you may run into problems, leading you to having to go into your BIOS settings and tinkering with the RAM settings. I'm sure you don't want to do this, so it's better if you purchase RAM with properties that match.