Depending on your motherboard, i would definitely upgrade your RAM. 16GB is getting to be the norm for a half decent gaming PC now. But im sure you would manage on 8GB, with low-medium graphics settings. 4GB wont be 4GB in the game as your o/s will swallow up 1.5-2GB for starters. So you would be trying to play the game on whats left!
Hope this might help you decide the best!
Kenton.