An online invite reads "Dice invites you to the unveiling of Battlefield 4. Please join us to celebrate a new era of Battlefield". As of yet, nothing aside from the numerical title has been revealed of the game, along with the fact that Medal of Honor: Warfighter pre-orderers will each receive early beta access to the game.
Other has speculated that members of retail has been shown some of the features of the game. But everything should be uncovered on March 26th.