I fired up my Silent Hunter IV game the other day and installed the MOD called “Fall of the Rising Sun”.
What a great mod! Honestly, it is the way the game should have been when UbiSoft released it. I believe it’s being actively maintained over at Subsim and if you’re interested in it you can go on over there to download it and give it a whirl.
You do need to have a Subsim account and you also need to have SH4 Gold Edition – that is the edition that has the Uboat missions included.