What happens if the ship’s natural cannons were a 5 or 6? (There are a handful in the game.) Do they reduce to 4, or stay where they are?
An easy fix would be to say “…all cannons become rank 4” so you’re covered either way. Better yet, you could word it like the range-extending abilities and say “…you must roll a 5 or 6 to hit” so rank becomes irrelevant.
Is that last sentence serving some specific purpose?
There’s no mechanism anywhere in the game to allow a ship to increase its number of masts beyond the number it started with, so as written, it seems to imply that if you ever repair the ship back to full after taking damage, you can never repair it again if it takes new damage.