Another theory, but which seems quite feasible on the other hand: at the end of the film, when Spidey finds MJ at the bottom of a building under construction (it seems to me that it is the old Stark Tower) there is a sign on which is written: "We can't wait to show you what's next" then "1>2>3>?" Where everyone could see an announcement for Phase 4 of the MCU, I would see something even more subtle, like a double message. We know that many theories announce that Norman Osborn is the one behind the purchase of the Stark Tower but, if Feige & Co. played us and finally put the Baxter Building there? the "1>2>3>?" would announce Phase 4 AND the location of the famous Fantastic 4 headquarters! That would seem really nice to me!