ORIGINAL: ChuckW
Generally, most people consider that a 4-stroke needs to be 1.5 times larger to get the equivalent performance of a 2-stroke. Using that, the .52 would be roughly equal to a .35 2-stroke.
That isn't a set in stone rule though. It depends which 2-stroke and 4-stroke you are talking about. They are not all created equal. For instance, a general rule is that you need a .70 4-stroke to replace a .46 2-stroke which very often is the case but I've seen and flown 40 size planes with good .52 4-strokes or the OS .56 2-stroke though and they performed great. Something like a YS-63 will perform as good or better than something like a .55-AX. Experimenting with props on a 4-stroke can get some very nice results to.
So the 1.5 times the displacement is a fairly good rule of thumb but not true in every case.
I'll go along with Chuck here. A .60 two stroke = a .90 four stroke, in general. Engines keep improving however. The OS .55AX is perhaps a bit more powerful than the old OS .60FX and their new .82FS-a is about equivalent to the old .91 Surpass four stroke.