Electronic Arts has let slip that players who purchase Medal of Honor: Warfighter on Origin will gain access to Battlefield 4's beta. Bear in mind, of course, that EA hasn't even announced Battlefield 4 yet. I guess this counts as your official notification.
Since the story broke, Electronic Arts has removed mention of Battlefield 4, most likely due to that little problem of having not announced it yet.
The idea of Battlefield 4 is hardly a shock, but methinks EA would prefer a more ceremonious announcement. If it all pans out, however, it seems like EA has a nice little Call of Duty thing going on, with Medal of Honor and Battlefield playing the Treyarch and Infinity Ward roles. Coincidence? Given EA's tendency to ape, probably not.