As previously reported, the likelihood of Robert Downey Jr. coming back for the role of Iron Man depends solely on how willing Disney and Marvel Studios are to pay the increasing paycheck for the actor. According to a report from Screen Rant, there is also the fact that Downey Jr. is getting old and this may impede his likelihood to reprise the role.
According to the report, Downey Jr. did address his age back in early 2015 when he admitted he would be roughly 50 years old by the time he would start advertising "Captain America: Civil War." His contract also ends with the aforementioned film and his appearance in "Avengers: Infinity War" is likely a favor to Marvel Studios.
The report also confirms that Marvel Studios president has confirmed that they are willing to recast the role if in case Downey Jr. is no longer available or if a new contract cannot be formed between the two parties. As previously reported, there is even the possibility of Ty Simpkins taking over as the man behind the iron suit.
However, there are also rumors flying about that the mantle of Iron Man wil pass on to other characters in the MCU. One of the most highly rumored members of the Avengers to take over as Iron Man is the War Machine aka Colonel Rhodes (Don Cheadle).
Rhodes has been wearing a variation of the Iron Man since "Iron Man 2," and in the comic book source materials he did take over as Iron Man for a short time. If the role of Tony Stark (Downey Jr.) is not re-cast then it is likely Don Cheadle's Rhodes will take over instead, with Stark only appearing as a guest advisor in future films.
"Iron Man 4" is currently expected to release sometime in 2019 or later, presumably after "Avengers: Infinity War" parts one and two. However, with no official word from either Downey Jr. or Marvel these should still be considered as speculations.
