I think all the bases where covered to some degree in most of the posts. 4s's are heavier, larger, cost more, different to adjust and require more maintenance. Most produce power at lower rpm and can swing a larger diameter prop. IMO the OS 91 fx should run no larger prop than a 14x8 but the OS 91 surpass can swing a 16x6. 2 strokes do not usually like a big load and can get real hot. My 4 strokes sip fuel better than my 2 strokes. Note that the ST 3000 is designed to spin a large prop at lower rpm the ST 2300 a small prop at higher rpm and your pay for power in the YS by cramming more fuel into the chamber with cuts fuel economy. I prop my 4 strokes to run at about 9k on the ground but most of my 2 strokes at better than 10.5k. By all means try one but maybe your third or fourth engine. Make sure you run a 4 stroke a little rich - you don't want the prop come off from a lean detonation. 4 strokes do sound great. Each type of engine will have different characteristics.