From what I've read, the rule of thumb for comparing four stroke with two stroke is taking the plane's suggested engine size, for example .40 two stroke and multiplying it by 1.7. Of course this is a very rough estimate and you'd be better off by looking at which props they will swing at how much RPMs. Note that with a four stroke you'll gain response from the engine but will sacrifice a little of top speed. Not to mention the fuel efficiency of four strokes, their real-like sound and their price $$$