Hi!
I use a 15x4 APC in my Great Planes GEE BEE R-2 powered by a MVVS 15cc engine (.91).
It works a little better than the standard 14x6.
14x4 is too small for models weighting around 5kg.
I even use a 15x6 APC in my U-Can-Do .60 with same engine.