RE: 4 stroke, for speed planes?
Weight is always an issue with a "go fast" plane. "Power to weight ratio" comes to mind.
Speed on a forty size plane I have had real good luck with a Rossi 45 with a mini pipe. Keeping the plane lighter. A four stroke engine in that class I could never get any speed out of it. The added weight was not worth the trouble or hassle.
Does not matter how many rpms you get or how much thrust. If the plane is heavy it makes it more difficult to make it go fast.
A five pound Sig Kougar with a Rossi 45 will get you in the 115 mph range quite easily, a five and a half pound with retracts 120 mph easy. That is where the added weight becomes an advantage because you reduced drag. A four and half pound electric Kougar can get the same speed. Both using a 10x6 prop.
Food for thought.