Firstly, 4gigs of RAM is not even nearly enough, unless you shut down your APU's GPU, it will stick consume RAM for its operation, eating away even more of it.
Secondly, Is your monitor cable plugged into the correct slot? You must put it in below, where you installed your graphics card otherwise it will run via APU.
Thirdly, when you installed your graphics card, did you plug in your power cables into the card?
Fourthly, 3.0GHz of your A8 may not be enough to run battlefield 4 by anything other than 720p.