Source - Assassin's Creed 4: Black Flag - Steelbook: Games + Assassins Creed IV Steelbook - Collectors Edition Forums

So it looks like Assassin's Creed IV is gonna have 2 Steelbook designs.
The one from the Skull/Black Chest Edition and Pre order exclusive steelbook

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Note: You can buy the steelbook above for 19,95 on, but those pre ordering ACIV ad will get it for free.