Phase 4 is the final phase of their beta, the one where they let the flood gates wide open. If you have ever signed up for an FoM beta account, your account has been activated and you can log in to play the game. All you need to do is download the game client, which is available through links provided in your account activation e-mail.
Face of Mankind, for those unfamiliar with the game, is a single server MMOFPS predicated on social conflict and politics between the game's eight player-controlled factions. The factions act like permanent guilds, as you must immediately choose (and stick with) a faction right from the beginning of the game.
Factions create missions for their players, all of which go to serve the faction's interests and reward the player with cash and experience, but pit factions against one another in both social and physical battles. As players level up their character, they can also choose to proceed to rank up in their faction and get the honor of controlling the way the faction runs and interacts with other factions. Through all of this, the game tells a story and provides a constantly changing political backdrop for players to engage with.