Im thinking of the SIG 4 * 60. I too have been flying the trainer, and my mentor has the 4 * 40, 60 and 120. All with 4 strokes. That plane is awsome. Itll do anything in the air you want, at high speed or low speed, plus itll slow down enough for a great landing.
Those 4 strokers are nice to..they sound good, have LOTS of power, easier on fuel, and not as messy as the 2 stroke.