here is no reason that the 4* 60 would not fly well with a 70 4 stroke. It has the same 72!QUOT! wing span and weighs less than a Sig Astro Hog which will fly quite well with a 70 4 stroke. With all honesty once you get it in the air a Cub is gentle flying airplane while there may be better choices for a second plane (because of ground handling problems) it still should not be a big jump for some one with lots of trainer time. You need more practice on your trainer, if you are having problems flying the Cub you will have problems with the 4* as well.