I've had the ST 51 on my 4*40. Lots of vertical performance and speed with an APC 11x5. This is a great combo. This engine with the APC 12.25x3.75 has plenty of thrust to hover and pull out, my ability to do so limits hovering. I don't think the 4*'s were designed with hovering in mind. Ground clearance is a little limited with the big prop, I've broken plenty. I have more fun with the 11x5.
have fun with whatever you get, Mike.