Level 3-4 has got two entrances. You seem to have found the main entrance. The second entrance has the boss key. It's on screen 7F (buttom right corner), bumbable wall. The main entrance is on screen 6E, diagonally one above to the left of it.

I don't know if anyone checks these anymore, but I've been enjoying the quest a lot. I believe I've completed all of the non-level 9 dungeons except for one: 3-4, the Devilfish. I've carefully combed over all every screen available to me but I'm unable to locate the boss key, which prevents me from obtaining three items inside. I believe one of those is my last triforce shard and most likely the final arrow upgrade, leaving me incapable of progressing further.

It's possible I could be missing something, but I'm doubtful because I've scanned every screen with a lens to ensure I didn't miss any secrets along the way. I'm thinking the boss key was placed beyond the boss doors. All randomization settings are on.