Latest released version is v1.5 which is only(?) available from U-Boot Missions expansion. Silent Hunter 4 (SH4) is American Silent Service submarine sim on Pacific Ocean against Japanese.
SH4 also has a lot of mods created over the years.
Since the modern Silent Hunter games 3 and 5 are both U-Boat stuff, SH4 is nice change where you get to play American skippers in Pacific. This game has odd movie grain type effect applied to it, you can decide if its good or bad.
Purchase SH4 from gamersgate, digital download and the version is Gold Edition which includes U-Boat missions and v1.5 patch.
Note: when the above link goes dead just go to gamersgate.com and search for "silent hunter 4 gold edition".