To me it is not simply about the maximum HP that you can get out of an engine, it also depends on what kind of flying you're after, so it is not that easy to give an exact "conversion formula".

I do not run 4 strokes myself but for sport use I think a modern 4 stroke 30 can replace a 2 stroke 25. The advantage is a more comparable weight and a larger prop swung at lower revs, i.e. a well behaving sport plane. If you looking for maximum power vs weight then I think you are better of with a 2 stroke anyway, at least in these comparatively small engine sizes.

The weight penalty of a 4 stroke 40 engine might make it fly like a brick while giving comparable power to a 2 stroke 25.