Oh my... an original disk version? I had one of those, and when I lost it, I was not very upset... lol - It was almost as bad as SH5... or SH3... or SH2, when they all first came out...
Give this a shot:
Silent Hunter v1.4 Update
You will have to empty the Save folder (C:\Users \UserName \Documents \SH4), then put the new executable in your game folder, run it, and it will update your version to v1.4, which is much much better, though there are not very many mods left that run on it... If you want to use some of the "newer" mods (after 2008), you'll need a v1.5, aka: Gold Edition, with the U-Boat Missions. Ubisoft sells it as a software download still:
Silent Hunter 4 Gold DLC
Both Ubi and Steam still have it, and for usually $10US, but it will go "on sale" for $3US sometimes... and Labor Day is coming up soon...