ORIGINAL: MinnFlyer
A 70 4-stroke won't be enough for a 60 size airplane, but it would be perfect for a 40 size. You never did mention which 4* you have (40 or 60) but if you have the 60, then a 91 4-stroke is what you would want.
Just plain WRONG.
A .72 4-stroke puts out as much (or more) power as a .61 2-stroke of equal quality. The 4-stroke will have MORE static thrust with the CORRECT PROPELLER.
A 4*60 can hover on a .72 4-stroke.