JT,

A four stroke will throw a bigger prop than a 2 stroke allowing more thrust to be produced. A 65/70 four stroke would be plenty of powe plus some for your 4-star 40. As I let you know already my saito 65 pulled the GP Venus arf with authority. I was able to go right into knife edge from take off without any problems. I believe the venus weights more than the 4-star also. The 65 would be just right.l

Rich