As you might have realized, some users are having troubles on the latest 9.0.4 version with the Tier 4 skills.
Here is some information about the situation and the upcoming steps:
Right now, if a monster tries to learn a skill of Tier 4 without having the Rank needed, an error will be prompted in the game. This error will be fixed in the upcoming 9.0.5 that will be live next week.
Some players had already equipped incorrect Tier 4 skills on monsters that should have not had them equipped. We will revert those skills and they will be removed for those monsters, applying the correct ones (Tier 3). This process will be run also the next week.
We also noticed that Tier 4 skills can be equipped for monsters in Friendly Live Battles. So far and since this is a test mode, we don't plan to remove this option for now.
Some monsters on Live Duels cannot be equipped with tier 4 skills, or if they already had those skills and we swap for others, they cannot be equipped again. This is already fixed and monsters inside Live Duels are now with Level 120.
Sorry for the inconveniences and thanks for your collaboration and understanding.