Hey Jerry! Good to see you over on the "dark side"... lol Most of the mods you will find are for SH4 v1.5 (Gold Edition - with U-Boat Missions) and are US Pacific-based, but if you do not want to wander too far from the U-Boat side of things, then you want
Dark Waters, which is what Fifi did to lurker_hlb3's Operation Monsun mod. Maybe you might want to try Niume's reworking of
Knights of the Sea Depths II:Ace Edition. Both are world-wide mods with the player driving U-Boats. Both have their strong points, and their not-so-strong points. Both are similar to SH3, but with SH4 "updates" that offer a slightly better gameplay, but SH4 does lose that claustrophobic feeling of SH3 somehow. "Atmosphere" also maybe??
For the US side of things, there is
Fall of the Rising Sun Ultimate (FotRSU), which is based on the AOTD_MadMax team's Fall Of The Rising Sun (FOTRS) mod, which in turn, is based on TMO v1.7 (Trigger Maru Overhauled) - I'm using base terms to describe the mods though... FotRSU is a "pretty" mod that attempts gameplay for realism... ?? It has a rather large Roster of ships, planes and submarines. Another choice would be Bubblehead1980's reworking of TMO in
Trigger Maru Overhauled Update BH, which is based on TMO also. TMO was originally a difficulty mod, but Bubblehead1980 liberally borrowed from other mods, including FotRSU for a larger asset Roster. There are some issues with CTD, apparently most likely due to the use of an add-in mod based on a "EAX_Clang_Splash TMO" combo. I have not had time to play the mod lately, and have not tried to find the issue. Then of course, there are the original mods of OpMonsun (lurker_hlb3) and Trigger Maru (Ducimus), both of which have a lot of possible add-ins and various version available. The
Run Silent Run Deep Campaign (RSRDC) mod is an add-on for both of those and / or the Stock game, and has a lot of "historical" groupings in the campaign files. Another mod you might want to consider is
Real Fleet Boat (RFB), which the Stock version of RSRDC can sit on top of. RFB attempts realism in the submarines' mechanics, and you can die easily, especially in an S-Boat, in which a rusted hull is simulated. You might make it to test depth, then again, maybe not... lol
There are other mods out there, such as lurker_hlb3's We Dive at Dawn (British subs, but not quite a finished product), among others, that are not US or German based. Whatever you do, use
MultiSH4 to create new, unique Save folder names, and use multiple copies of your SH4 install for the different mods to help you sort through them all. Most of the newer mods will require the use of the 4Gig Patch or LAA to gain access to more RAM.
