I would give it a run trough Motocalc so that you can be sure of the numbers before making the investment. It makes sense using a 4000 4S, it should give you around 12 minutes of flight time, and enough power to tow. But you'd better make sure. If you plan on flying electric planes often (Which I think is the way to go) then Motocalc is a great investment.

Regards,

Javier