Why is the german version only for 4-8 players while the english one is for 2-8?
Where did you see that?
The Pegasus website is showing '2-8 players'.
EDIT: Nevermind. That's a link to the English language version sold through the Pegasus webstore.
I can see what you mean in this video: https://boardgamegeek.com/video/124856/captain-sonar/captain...
The box does show 4-8. My first guess is that the German version wouldn't include turn-by-turn, which the only way to play it with less than 4 players, but the video includes some of the turn-by-turn maps.
The German edition of the manual also covers turn-by-turn, but the Team Lineup section does stop at 4 players.
In short, I can't answer your question. Maybe email Pegasus?