Just like low@slow said, a .52 4-stroke is plenty of power for the 4*40.

I have a 4*40 that's been busted and repaired a couple of times, so I know it's a little on the heavy side. I have a Magnum .52 4-stroke on it and it goes like a bat-out-of-heck! Plenty fast and it will do any aerobatic I ask. Spins, snaps, knife-edge, whatever. I haven't figured out yet how to hover, and it might not have quite that much power, but I'd be afraid it would break it with a bigger engine.