I agree with bigshootr8. 8 GB is what I recommend as the sweet spot. Make sure you're running a 64 bit operating system. 32 bit operating systems usually can't go past 3.1~ GB of RAM. Without a 64 bit OS the rest of the memory will be unused.
If you aren't running any programs in the background, then more RAM isn't needed. If you are running things in the background, such as email, youtube, music etc. or like to minimize the game to do something, then the extra RAM will be useful.
You can always open Resource Monitor and look for 'Hard Faults' in the Memory section (while gaming). Hard faults are caused by not having enough RAM, and can greatly slow down a system.
I'm not sure if you'll actually get a lot of in-game performance increase, but with RAM being so cheap nowadays I would say go for it, just for the sake to be future proof for upcoming games.
One effect that 8GB will have over 4GB is to increase Windows performance, I remember reading a technical article some years ago where they tested two identical systems and the extra RAM made boot time faster and some other OS functions as well. For instance, you might notice that jumping from 4GB to 8GB will make Windows consume an extra 200 MB of RAM on idle after booting, this is because the OS will cache more stuff in memory thus accelerating certain features.
One thing that you must absolutely verify before upgrading is to make sure your OS is 64-bit otherwise it won't support the extra memory. If your Windows is 32-bit, you will have no choice but to reformat and reinstall a 64-bit version. Because 32-bit has a limitation of 4 GB or less.
Also if you go for it, I would suggest you buy exactly the same RAM module your currently have (OCZ Platinum 4gb DDR3). Not that you will necessarily have problems if you buy a different brand or model, but it's a little safer when all your RAM modules are identical.