Why are tier 3 submarines stronger than tier 4?

• Why do tier 3 submarines have 52 attack while tier 4 subs only have 36 attack?
• Is the production cost alone enough to make tier 4 better than tier 3?

Solution 1:

Note that there are more stats that matter. Surface Detection is higher, it's slightly more reliable, durable (HP) and has a larger maximum range.

While Torpedos are useful in combat, submarines are specialized units not meant for actual combat and as such more advanced submarines are specialized on finding enemy units and raiding convoys (which they can even with lower torpedo damage).

All of those stat changes and the production cost being almost halved mean that you have a much more efficient unit for recoignassance and raids, but also mean that even if you want to keep fighting with submarines, you won't get that much worse because even though your submarines do less damage, they can take more damage and you'll have more of them.

Solution 2:

Do you have Man the Guns DLC? If yes, then the reason can be in different modules. Newer submarine might be using outdated torpedo modules, or have some empty slots.