My first 4* had a OS46AX and it did marvelous. Lost that plane due to disorientation while inverted. The 4* I have now has the OS 50SX on it and it absolutely screams across the sky. The 46 is more than enough and the 50 is mega wild. I bought the 50 as I thought it might give me more versatiltity in other planes. The 46 is around $115. and the 50 is around $150, so theres a fair amount of difference in cost. You'll find many opinions about engines on here but MOST will agree that OS is top quality, albeit at a premium price.