I have owned two 4*60s and flown several others and here is my assessment.

2-stroke .61, full wing: Sort of pokey, not much vertical, slow on rolls. Flies like a big, low winged trainer.

2-stroke .61, clipped wing (1 rib each side): Still pokey, but faster on rolls.

4-stroke .91, full wing: Good hi-performance sport plane, good vertical.

4-stroke .91, clipped wing: Better due to faster rolls.

2-stroke .91: Most power, excellent vertical, best with a clipped wing.

My 2 were kit bashed into a "sort of" Kawasaki Tony with clipped wings, rounded tips, scale tail and gear mounted in the wing. One had an OS .91FX with an Ultra Thrust muffler. The other had a Magnum .91XLS with an Ultra Thrust muffler. Both used 14-6 APC props and both turned 12,000 rpm. I could not ask for a better sport/acro plane.

If you are interested in the Ultra Thrust, call Performance Specialties. They make two for a .91 2-stroke, a short one for running .61 size props faster, 12-6 at 14,000+, and a long one for the 14-6. They really like to rev up the OS .91.