Maybe it would help to install SH IV on another drive?
After opening the data\config file in the Sh IV folder, I encountered a lot of references to SH III, German named crewmembers, German subs etc.....
I have absolutely no clue how they got there, I mean SH III is installed on a completely different drive.....
there is definitely some kind of mixup here, but SH IV runs smoothly...patched and all.
Anyway, I'm not sure it will help, but you could give it a try.....
Good luck!