Foxy

I'm running 15t on the same 2200kv system in a mugen truggy (more weight and slightly bigger wheels) on 6S and getting half an hour before needing to cool (at 160F). It's a very strong system. I would say in a buggy on 4S, you could start at 16t and probably have quite a bit of room for movement. When I get an e-buggy (which I will soon), I'll probably run 17t.

Really, heat on 4s with that system is not something that you will have to worry about a lot, at least not with logical gearing. I would say focus on what gearing is best for the track and worry about heat later. For example, I don't think you'll have any major heat issues on 17t, but if your track is quite technical, it may not give you the desired punch out of the corners.