As others have stated, it really depends on your goals. If you're looking to push higher into M+, I'd certainly look into tank + BM x3 + healer comp. If you're looking to kinda mix it up, I had quite a bit of fun mixing my teams and running BRM + MW + Boom x3 where I would swap the Boomkins for BM as I required. I'll admit that I'm certainly no authority on the subject though as my highest multibox key has been a +6, even though I could have probably pushed higher, quaking last week really scared me. lol

You mentioned the 4x DK + Pal and while it's interesting, I probably wouldn't recommend it, especially as your first team. The growing pains of an all melee group, especially an all tank [read: lower dps] team has all but put me off it, personally. Obviously as gear increases, as well as corruptions like Twilight Devastation, things will become easier but just be aware of this fact.