BF4 is likely too processor-intensive for the core2quads. Both my buddy and I had a Q6700 and Q9540 respectively, both heavily overclocked @ ~3.5 GHz - we ended up being bottlenecked significantly and stuck at <40-50 fps at low settings across the board.
Needless to say, we're both running Haswells now
1 - Graphics settings have very little impact on whether a CPU can handle the game. If you had a Titan Black, you'd still be at 40-50FPS on Ultra as with low.
2- 40-50FPS is certainly playable. Haswell makes things better, but a Q9650 is workable for BF4 for sure.
You can get an AMD videocard and bring life to that CPU specially on BF4 since you get Mantle. A 270X/280 should be the perfect match.
My old Q9450 @ 3.6GHz got <40FPS (and averaged more like <30fps) on 32 player servers during beta. I didn't even attempt a 48 or 64 player server after the first few tries yielded terribad results. GPU settings didn't matter because the CPU was the absolute bottleneck.
Convinced me to upgrade my CPU (now a 4700k and I get 60fps)