I did a lot of research regarding the LT-40 and ideal size engines. I had seen a few with 4 strokes but don't recommend them for beginners. Cost is the primary reason - they cost up to 3x as much as a comparable 2 stroke.

I chose the Thunder Tiger .46 Pro. It's a good performing engine you can get for around $80-90 bucks. ABC construction and ball bearing supported crankshaft. It weighs a bit more than an LA .40 or LA .46 but this helped me as the kit-built LT-40 tends to be a bit tail heavy. Mine balanced perfectly with the TT .46 Pro. While it doesn't overpower the model, it gives a bit of instant thrust to get quickly out of a potentially bad situation.

If I had to do it again I'd go the same route again. Perhaps that's why I bought 2 of them.