A lot depends on the type of plane, and the performance you want. For example, a "40 Size" Piper Cub will fly nicely with a .52 4-stroke. But if you want you Cub to do some nice "Clipped Wing" aerobatics, a 70ish 4-stroke would be better.

For MY personal taste:

.46 2-stroke = .70 4-stroke

.60 2-stroke = 90ish 4-stroke