RE: What is a good engine for a four star 40?
The AX is an excellent choice. My personal preference would be for that lovely TT 46 Pro -- but hey, that's my preference, not yours.
Essentially any 40 - 50 size engine will fly a 4* quite nicely -- even the relatively weak OS 40 LA.
If $ is an issue & you'd like to keep some of it, try a TT 42 GP, or an OS 46 LA -- both work really well on a 4* & both are pretty much dead even in performance.