Iron Man 3 had a handful of new characters that we were introduced to, some good, some not so good. If you think back to the kid in the movie, you’ll remember him as Harley Keener, the delightful Tennessean who helped Tony find the dastardly Mandarin. Keener could be returning for a fourth installment if Marvel decides that it will work well with the plot. According to a string of sources, 13 year-old Ty Simpkins may be taking over the Iron Man character as soon as 2019’s Iron Man 4, which will be post Avengers: Infinity War Part 2.
Robert Downey Jr. brings a lot of character to Tony Stark, but will he still want to rock the iron suit after The Avengers: Infinity War? Only time will tell, but there is a chance that he could pass the baton to Keener or to another young actor. RDJ’s contract ended after Iron Man 3 and there are a few other movies that he will be making guest appearances on to solidify the Avengers team. He will also be stopping in to help Captain America: Civil War. Behind closed doors, there could be negotiations leading to another contract, but only time will tell.
This speculation should be taken with a grain of salt, since Marvel may still have some cards or missiles up their sleeve.
Source: Movie Pilot