The general graphics look
worse than SH3? That's a rare comment around here. My main reason for playing SH4 is how much better the graphics look.
One thing I would definitely recommend is Operation Monsun. It gives a full campaign lasting the entire war and all the subs available in SH3. We actually have a full forum just for the German side. You'll find out a lot more by reading there.
http://www.subsim.com/radioroom/forumdisplay.php?f=232