I have made a comparison among the recommendations for 2 and 4 stroke for several of the ARF's and kits commercialized by Tower Hobbies.
The rate is about 30% more displacement up for an equivalent 4-stroke.

In this case: 0.52 / 1.3 = 0.40

Since you guys have plenty of experience with these engines, let me know of any inaccuracies you believe my chart may show.