I perfer a .50 size engines becuase you'll never what model you might want to get later.
This way i don't have to worry about arf that are bricks, and if I wanted to install retract or floats, being under power won't be a problem.

The 4* .40 is .32-40 range originally. A .46 will pull it striaght up if you wanted to.

I'd be very careful if installing a .50 becuase the way the wing is designed or constructed. I'm not sure exactly how a 4* arf are consrtructed.
Even with a kit, I still build a little different.

The shear web are installed between the top and bottom spar....not a problem if you build it correctly or make sure there's no gaps.
I opt to put extra shear web by install it in front or back of the spar.

If you had ever had a wing rip away before..It's mostly from the wing joint seperating from one of the spar and ripping through
the opposite spar, from not enough epoxy or the epoxy didn't have enough hardener in it...Therefore cuasing the seperation.

There's so much force apply to the wing , it'll either try to compress it or rip the spars apart.

That's why the Fokker DRI Tri plane was call De flying coffin.lol It didn't have any main spars.