It depends heavily on which task you are planning to do with your PC.
Note that, if your current OS is 32bits, there's no point in adding more than 4GB of RAM because it won't use it.
But if it's 64bits, there's no problem in adding more RAM.
If you only use it for browsing/working that should be enough if you don't open a lot of tabs/files at the same time.
If you wanna play games, most of them won't work properly since 4GB is not even the minimum requeriment. Unfortunately.
RAM is one of the cheapest PC part, try to get at least 8GB, it will be much better than just 4GB, for sure.