I forgot to answer your main question. If I was to decide upon a four stroke engine I would choose one with the higest power to weight ratio. For a 40 size plane three are the engines with the best p/w ratio. These are:

YS 63, Satio 82 and OS 70 Ultimate.

If I was to decide now, I would go with the YS because I you can put the tank at the CG range. Saito is not bad though. Do not use 91 or 100 it will come very very nose heavy!!! You will have to put lead at the back and the plane will be 3Kgs in the end!! Simpily DO NOT DO IT!!!

Don't go with the Ultimate, good engine in my opinion but is needs lots of nitro and does not have the same output as the YS, it is also the most expensive. I bought ii to use it with less nitro but I can't, it doesn't work. If you don't want the YS because of the high cost get the Saito 82 without any further thought.

By the way here's a photo of my flip.

Nik