As can be seen from SteamDB, Half-Life 4 was added today to Steam, well ahead of the Half-Life 3 debut. The Half-Life 4 entry also hints at binary support for the Steam Box, Valve's forthcoming console. The Half-Life 4 entry also notes use of "Source Engine 2" for the game.
Before getting too excited, however, the PreloadUnlockTime is taking place on 1 April 2013. While Half-Life 4 this year is an April Fool's Joke by the Bellevue company, their Linux-based Steam console for this year is not. It's been one year they have been pushing hard on Linux.