Monk starting gear this season is a really strong set. if you go with 4, funnel all the non set legendaries to one monk at a time to get them geared, one fully geared monk with crap gear can carry the other 3 quite easily.
As far as the expansion goes, I'm pretty sure that is how you get to level 70, otherwise your stuck at 60, and can't do greater rifts or any of the stuff that makes the game fun.