First of all no matter how big the engine you put in a plane, it will reach the point that it will fly only so fast. And a 90 is way to much engine for the 60 4*. Don't get me wrong you can use it but is too much, and why add the extra weight. I see what the max suggested power the manufacture recommends and go with that. Second the design of the 4* 60 and how the wing is framed up has a twist factor if you exert to many Gs on it. Is not a D tube construction. Is the reason they tell you to use film rather than fabric to cover the wing with, to arrive at a certain tinsel strength. You can overcome this by copying the inner ribs in front of the spar to allow for a 1/16" sheeting, thus creating a D tube. Will negate the twist/tork of the wings. I fly with 4 different men that fly 4* s as i do and the most we put in them is a 91 4 stroke. And fly it with that at half throttle. A 70 4s will fly it very nicely with plenty of power. The 90 4 stroke will give you unlimited vertical but remember this is not really a 4d type of plane even though will perform many stunts.

DiBo