I think a lot of time and effort is being misplaced in talking about engine weight. With an LT-40, so long as you're in the ballpark you're fine. It's a big airframe with a very generous wing size.

Four-strokes are nice motors. The quality is generally much more even than two-strokes. I don't know of a bad four-stroke on the market right now. The prices are slightly higher. They're really no more difficult to run, just different. A tachometer becomes a necessity, but that also takes the guesswork out of tuning. That may make it easier for a beginner. At any rate, it's a personal choice. It won't greatly impact how the plane flies, but it may impact the level of personal satisfaction involved.