It's fairly easy to have 8GB of RAM on a desktop, and old rules of thumb on swap partition size will give you a huge amount of virtual RAM, but it does depend on the software you are running.
So check what your computer is doing. There are graphical programs such as Task Manager and command-line programs such as
free -h (I use that form of the command because it's easier for a human).
There are programs which struggle in 4GB of RAM. It isn't just the particular program, there is a lot of OS code that has to be there, and when you go to 8GB, you can get a little careless about what's running. If you don't need to swap, use the RAM.
I have noticed the 4GB struggle with Kerbal Space Program and the Firestorm viewer for Second Life. There are alternatives to Firestorm which don't need so much RAM. Same task, different demands, that's why you have to measure.