I came here from XI myself and... I think 4 is plenty. 6 was a necessity in XI because of the nature of the game, if you needed someone other than the tank to sponge some damage you had the option in the other DDs. In this game combat is fast enough that that's not an issue. Most everything dies before the healer is too taxed or the tank drained to auto-attacks. The only reason to have more DDs is to trivialize the runs and, frankly, that's boring.