As mentioned above, there are no "official patches" that add new U-boats, only community mods.
Type-XXIII
There is a Type-XXIII. Initially this started as an AI-only mod, but it has since been converted into a full-blown human-playable boat. The main release was for NYGM, but some people (incl. myself) have managed to get it working in GWX-2 and GWX-2.1. (I don't know about Stock 1.4b, but I don't see why not.)
http://www.subsim.com/radioroom/showthread.php?t=116988
Type-VIID
There is work underway on a Type-VIID. Some of the parts such as the late-war external conning tower are now done (although Tomi_099 is still working on perfecting an early-war model). I'm currently working on the menu graphics for the game as well as doing some investigations on getting mines to work. This is work in progress, and it is not ready yet.
http://www.subsim.com/radioroom/showthread.php?t=120420
Other boats
I don't know of any other
human-playable German U-Boats. There are some mods floating around for Schnellboote and I've seen one for an Admiral Hipper class (never tried it though).
There are some "cosmetic" mods, such as Type-IIB and U-Flak, but these are essentially the same as what you can get in the stock campaign, but just with the labels changed to make it a bit nicer. They don't really add anything new, that you couldn't already get in the stack game.
Non-German Boats
There are also some mods that provide some other playable boats, these are available from here:
http://www.ebort2.co.uk/ (I've not tried any of these!)
I hope that helps.