The Fourth Seal contains Diablo the Lord of Terror.
Diablo has a CD of 1, however:
-Instead of attacking every second time, he will change his attribute to a different one.
The order of change is Light->Dark->Water->Wood->Fire.
All other attritubes other than the opposing attritube will deal only 1 damage.
-He does 6180 every time he attacks, so plan your HP and Recovery accordingly.
Diablo will be a test of your teams survivability more than anything else, as you will need to sustain 6180 damage every 2 turns and his HP is incredibly high, so bursting him is not a viable strategy except for the best teams. Low CD Active skills that increase survivability help a lot in this fight, such as Great Schemas.