The string of numbers? That's a stand-in for "June 24, 1459," or for you Auditore family historians, Ezio's date of birth. And the Ottoman Tughra? Turns out that's an official seal or signature for a sultan, used to sign official documents and for money printing, among other things. It's like Prince's symbol, basically, except it originates with the Ottoman Empire rather than an egomaniacal musician.
Anyway, between the major landmark (Hagia Sofia) and the Tughra, we're guessing the big reveal will somehow center on Istanbul, potentially in the mid-1400s. As for when we'll find out, your guess is as good as ours -- the meter towards the next teaser is at 20 percent as of now, with no indication of how many unveilings are left in Ubisoft's plan.
Update: The fifth teaser has now gone live, and it features a man who looks suspiciously like Assassin's Creed 1's main character, Altair. It also features a two-headed eagle -- a symbol quite popular in the Byzantine Empire -- and the number string "11011165," which is presumably a placeholder for January 11, 1165. What happened on the eleventh day of January, 1165, you ask? That's a good question. Other than some unexciting interactions with Russia, not much was happening in Constantinople/Istanbul that day. Perhaps the next teaser will clue us in!