You'll be more than fine with that rig.

I run 5 accounts using an i7 920, 12 GB RAM (six should be fine), GTX 260, SSD. In 5 man instances my main gets 50 FPS because I cap it at 50. In Dalaran my main gets about 35 FPS. My slaves are always capped at 10 FPS. Make sure to load balance on your proccessor cores.

I'd suggest moving from 4 to 5 unless your goal is PvP.