I really don't know how a four stroke would work on Gas/Glow, but I suspect it could be problematic because the four stroke engines don't keep the plug going hot like a two stroke.

There are Ryboi four strokes in the 25cc size range, but I don't have any experience with them, though I have heard they are low on power. Then there are the Honda four strokes as you mentioned. They are however rather heavy, and the power is also limited. If you get one, don't get the 25cc nor the 31. Get the 35cc as it is a newer and more powerful engine and the weight will be about the same as a 25cc.

The best choice I think, though they are hard to find, would be a Stihl Four Mix four stroke engine. They don't have an oil sump and are therefore much lighter than the others. Power also is surprisingly good. They make them in 32, 36, and 65cc versions. I have used the 32 and 36cc engines and I like them. I sold one 65cc Four Mix engine to a guy, and it runs/flies great. Amazingly, it has about the power of a Zenoah G-62.

Once again though, if you try a four stroke on Gas/Glow, you will be into a new area of experimentation as I don't think any of us here has tried it yet. I am somewhat doubtful of the results.....

Good luck,
AV8TOR