Seems like the 4 position is defined today as either a Stretch Four, or traditional Power Forward.
When I think of stretch 4's, Kenny Kadji always comes to mind. He ran the floor well AND had an outside shot. Made him very hard to defend. Murphy was a traditional Power Forward.
For all we know, maybe Castro can fill a Stretch Four role. If he can't, and can't bulk up to play down low on the baseline, I wonder if he might eventually be a 5. He has the height, you mentioned the little hook shot, and he seems to block shots well ...