You can always try it with 4gb of ram and see. If it's not enough, pick up another 4gb stick. Using an apu you're already at a disadvantage compared to a dedicated graphics card. Having only 4gb of ram can also be a slight disadvantage. If you want to go cheap and compound the problems further by reducing your system ram, sharing some with your apu on top of the apu's limitations that's up to you. I just wouldn't count on enjoyable results. Mixing ram size and speeds/timings may either not work at all if they're incompatible with one another or if they do work, drag down the speed of the limited amount of ram you already have.
If all you care about is price, get the cheapest you can and however it plays it plays whether it's 30fps, 20fps or what have you.