I like where this is going very much, but I think there's sme banding happening fairly noticeably on the nose in the second head (also one in preview) - I don't have the zoom plugin on my browser to see for sure but that's the area that jumped out to me as a bit odd, but it doesn't seem to have been an issue on the other two faces c:
2nd one looks better than others, also the right side of his face looks too bright for me, I think if you put the shade intensity of it like the 3th portrait it'll be the best one.
good work.