Well, if this isn't a confirmation of Just Cause 4, I don't know what is.
First, we had the WalMart Canada leaks that reported a new Assassin's Creed, Rage 2, Just Cause 4, and a slew of other titles. Now Steam has leaked ad and key art from the yet-to-be confirmed title.
Kotaku first broke the news, and its not surprising to see this happening to Steam of all platforms, after the headline news they made this week when it comes to how they're handling their store submissions going forward.
No other information is available at this time regarding Just Cause 4, but we can now surmise that we'll be seeing this next week come the Square Enix conference.
Again, Just Cause 4 was leaked by Steam earlier today.