onlyb, the 70 FS is a great engine, but wouldn't be my first choice for either the 40 or 60 size 4*. If you put it on the 40, you will have to add tail weight to balance it. This means a hotter landing and a loss of nimbleness. I have a .91 four stroke on my 60 size, and I wouldn't consider it over powered. My friend put an O.S. 61 on his 40 size, and now he wishes he'd put a .46 on it. It will go like a bat out of hell though, and the 4* has so much wing area, a hotter landing is still pretty slow.