Who do I need for the 4th slot?

Since I only have 4 slots, I'd like to know what I should be training up. I have a DPS/Traptress (not sure which one I'm going to settle for yet), Frosty Apprentice, and am currently training up a Hearty Blockade Squire. I'm not sure what I should get for the fourth slot: Aura Monk or another Huntress.

generally the most common solo build would be trappest/waller/frosty and then either a huntress dps or a monk. its up to preference on whether you want a dps or a monk. 

If you go for a dps you rely on frost towers plus your huntress dps for anti air and you focus your attention on the hard lane with the wither beast while repairing the rest of the lanes. This is a solid approach because with the dps huntress you save DU on skyguard so you can get more frost towers or traps to deal with waves faster, if the enemy is dead it cant deal damage. Down side to huntress is you have trouble dealing with the phys resist lane sometimes. You generally have to have a couple of extra defenses in the phys lane if the phys resist lane is the hard lane, or you have to already be focusing bonus defenses there.

If you go monk you have more utility but you have to use skyguard for anti air which isnt as good as a huntress generally because of the huge DU sink and lower dps output. The plus side is you now have boost and serenity aura. Boost can be used to boost some key lanes that are much harder (like life root mid and maybe the right lane if needed) so that way you can more easily out repair the damage and your walls wont drop instantly if your not able to repair it at the time because you have to focus your repair else where. You also get the serenity aura for its 20% slow (which stacks with frost towers and slows also effect enemy attack speed so it also makes your walls tankier) and you can get purge evil on you serenity so that you get a little more damage out of your lane. The downside the monk besides having more trouble with air is that you also deal less single target damage, so it generally takes longer to kill ogres. But if you go monk you generally use the waller as your combat phase hero, so you can stall lanes easily.

This is my experience in NM4 solo and also from what ive seen others do. If you dont solo just go dps so your more useful to the group in the combat phase and hope one of the other three people have the missing class you want.