If you want German boats in SH4, give
lurker's Operation Monsun a whirl. While not as steeped in ships and planes as FotRSU, if ya gotta have Uboats, ya gotta have OM... There are about 10-12 mods that can all work together, though I used to just do OM705, the 20 update and the patch. I don't recall ever making it past 1944, and now I can't find my old download files... I archived a bunch of stuff a couple of weeks ago, and now they're like my SH3 and original SH4 disk, and done gone invisible on me...