Have you ever wanted to use a torpedo but you want it to hit the top of your target? this is a real phenomenon called submarine missiles or something. These submarine missiles would be like a vertical torpedo that go into the air then land around 50-60 blocks and they explode (like a torpedo but vertical) the way to balance this would be simple. Like in the Cold War and World War 2 you would have to surface all the way to fire these making your sub vulnerable to enemy attacks.
but 4 inch guns dont do as much damage the point of this is to add more options imo and a torpedo does much more than a 4 inch gun making it useful and torpedos can launch multiple very fast so thats an advantage plus u can bombard shores with this better than 4 inch guns
I think torpedoes are good enough for submarines. US Navy's Ohio-class submarines have 18 torpedoes, 1 TLAM, 1 hypersonic missile and decoy missiles. Not sure about others but torpedo limit can be upgraded. I would prefer they could only carry 16-18 2 sized torpedoes instead of 4 sized ones. Also maybe if they could lock to the targets, destroyers and bombers would have more important position in fleet.