I just managed to clear ?-4. All my units died except for one that barely survived. I managed to kill all bosses and all pests except for one pest which survived at around 5% (SO frustrating!).
My units had: 48366, 47995, 42466, 32834 and a helper with ~60000. If not for my dang 32834 unit not doing enough damage, I would have three medal'd the map. So I'd say somewhere around those numbers would be the minimum requirement for completing the map (probably less for clear only).
Edit: I followed the strategy posted in the original post, I had two units on the second tent, and three units on the third tent. This tactic should work as long as you haven't somehow angered the gods of RNG. If you have, do the same thing but pray harder, of course.