Again...John Blake was Nolan's homage to Robin. He isn't meant to be Robin. The first name is a homage to the character. It was a way to tip the hat without breaking his own credo that we would never see Dick Grayson in these movies. As pointed out Tim Drake discovered who Bruce Wayne was on sheer detective work, I'm guessing Blake worked that out on his own using the same methods as well and putting two and two together.
The end of the film very clearly shows that Blake has the coordinates tucked into to his duffle bag, thus he inherits the cave from Bruce.
As for a fourth movie, I would love to see one, but I seriously doubt that would happen. Warner Bros has an excellent working relationship with Chris Nolan. Nolan has stated quite clearly that he has no wish to continue this universe. Warner Bros would be rocking the boat with this relationship if they even considered doing something without first seeking Nolan or Goyer's blessing first.