If its a .40 size plane, I would say something like a minimum .56 four stroke and probably no bigger than a .72 four stroke which would give you plenty of power. Some where inbetween would be good though.

As far as diffrences between two strokes and four strokes, I would say the major diffrences are 1. the price (four strokes are way more expensive), 2. four strokes sound a lot better, 3. four strokes supposedly get slightly better gas mileage.

David