Just Cause 4 definitely exists, and it's definitely coming to Steam. We were led to expect as much thanks to a leaked Walmart document last month, but now an advertisement for the game has appeared prematurely on Steam.
Kotaku spotted the advertisement first (embedded above). Not long after, Wario64 posted the image on Twitter. According to the former, clicking the advertisement only leads to the Steam front page, and there's not currently a store listing for the game. The ad wasn't delivered to me when I logged into the Steam client, so it could have been removed already.
It's the latest in a series of pre-E3 leaks, most of which can be traced back to that bothersome Walmart list. Rage 2, Assassin's Creed Odyssey and a new Splinter Cell game were among the highlights of that leak, with Splinter Cell the only major title we've yet to have verified from another source.
Whatever the case, here is a handy list of the E3 2018 press conferences. Square Enix's conference takes place at 10am Pacific on Monday (6pm BST, 3am AEST), so we'll likely get confirmation then.