While i definitely want multiclass as a feature for release, i think playtesting lvl 5 and the remaining classes is more important right now.
In fact, playtesting lvl 5 is probably more important than hhe rest of the classes, because the jump in player power from 4 to 5 is huge. Underdark and Grymforge seem like they could be lvl 5 areas and should probably be tested that way.