Not overkill at all. Is one of the four probes you ordered an ambient temperature sensor? I'd definitely recommend getting one of those.
The setup I built with my iGrill 2 is a four probe set as well but two of them are ambient temperature probes and two of them food probes that I monitor with the old version of the app on an old original iPad mini that's been retired from other duties. It's set up so I can do WSM Upper Ambient + food, WSM Lower Ambient + food and that uses up all four channels right there.