When I was gearing my team up at the start of the expanion I would queue with just four. My win rate was very good. I'm not sure how it would go now and I have no desire to enter the battlegrounds until the whole automatic banning issue is addressed. When I want to cap area I do it in 2s running double dks, takes about 1-2 hours to cap.

It seems to me that heroics (first of the day) could be just as effecient using three or five since the speed of the run comes from queue time and the damage.
My main focus is running older raids and those will always be easier the more bodies you have.