The top-selling PS4 game on the US PlayStation Store is thatgamecompany's gorgeous petal-collecting game, Flower.
Granted, if you include PS3-to-PS4 'Digital Upgrades', the list is a whole lot more predictable; Call of Duty: Ghosts ahead of Battlefield 4 and Assassin's Creed IV: Black Flag. But remove upgrade downloads, leaving only PS4 games that were newly purchased via the PS Store, and the game that comes out on top is, rather surprisingly, Flower.