If the Windows OS on your PC is 64-bit Windows, it will be able to utilise more than 4Gb RAM – and you should stuff as much as you can into your PC. Be aware that you should takes pics of your existing RAM with you when you set out for your local computer shoppe, as there are many, many different types of RAM and not all can just sommer fit any PC or Notebook.
32-bit Windows can not use more than 4Gb RAM so don't waste your time and money on a RAM upgrade – rather upgrade your PC to 64-bit (and your Windows OS) first.
How to check?
Go to "Settings, System, About."