ARMA II runs like shit no matter what system you have and ARMA III is intensive so don't be expecting to run it on anything but low. As for BF4, you could MAYBE get away with medium if you bump that 7850 up to 270 as Doc suggested and make sure to get another stick of RAM, once again, as Doc suggested. If you're wanting to run games at a higher setting then I suggest that you get a 280x and leave it at that. Overall it's a nice little system that should last you for a year or so with current gen games.
Even 6 gb of ram would be enough but you might run into trouble with some newer games with only 4 gb. Also arma 2 and arma 3 are cpu intensive games so i think you should be able to run one if not both on high with that cpu.