I was thinking the fourth film should be the last. And a Logan type of film. Tony is getting old. He can't really use the armor like he used to against the new age of villains. A near death experience until he's saved by(insert hero cameo her) drives him him create an AI fucntion in his suit which battles for him and automatically picks out the best tactics in the suit so he doesn't have to do any of the work. This backfires on Ultron levels when Tony's AI turns on him again ala story "The Mask in the Iron Man" and it's up to Tony to stop it(with help) and except that he can't push the world away his whole life, he needs help, and he has to officially retire. Passing a new armor on to a protoge[Amedeus Cho as Iron Lad/New Iron Man please ] and starting a whole knew series of Iron Man films.