So far all my SH4 playing is in u-boats, using OM and parroting my SH3 careers. SH4 is better all 'round, except in the mods department. Still hoping for mods that match all the SH3 goodies available. I can't play fleet boats until I get to December '41 in game, and that may not be for years.
That said, SH4 is better visually and viscerally, and smoother as well.
Also there's a new u-boat mod for SH4, by some guy. Can't remember his name offhand, though.