Alnico 4 in a humbucker can give it more mids than a5 but also more bite to the treble frequencies. Personally, I think a humbucker needs to be specially wound to compliment the alnico 4 magnet. Lindy Fralin does that and his humbuckers sound great, but I have some alnico 4 magnets that I got from Lindy and when the guys here tried them in thier Duncans a few thought the alnico 4 made the pickup a little harsh. IMO, that's because the pickups weren't voiced for alnico 4 from the get go.
Lew