Still looking for that fourth secret. What I've found so far:
There's no light puzzles in the game as far as I can tell. I'd guess a light puzzle with the jack-o-lanterns to open the glitched grass floor if there was anything that could be a hint for it, but I'm not seeing anything.
You can up-hook the top of the screen in both ghosty forest rooms but not in other rooms. You can't crawl up after hooking in any of the spots I could reach, however.
The first ghost forest room, the one with a lot of ghosts, keeps playing ghost sound effects even after all the visible ghosts are killed. Perhaps there's one in a secret somewhere? You can hook the underside of the big block that divides the screen on the right, but again you can't climb up. The log isn't anything based on the last comment, but that's in the room, too. The crow in the boss room has sounds playing even when it's not there, though, so maybe this is similar.