It's not that surprising when actors from previous Marvel movies come back for another installment. It's even less surprising within the massive casts of Avengers: Infinity War and Avengers 4. According to IMDB there is an actor from Iron Man 3 returning to Avengers 4 and it's probably not who you think.
— IMDb (@IMDb) October 20, 2017
Harley was the kid from Iron Man 3 that helped Tony get his mojo back. He was one of the rare types of movie kids in that he wasn't the worst part of the entire movie. In fact some of the best lines from the movie are between Tony and Harley. It mostly comes down to Tony not treating Harley like a kid which made their back and forth feel much more natural. Why Harley is coming back is unknown though. We can hope it isn't just to give Tony a reason to retire if he dies.
Avengers: Infinity War, directed by Anthony and Joe Russo, stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Chadwick Boseman, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Don Cheadle, Tom Holland, Bradley Cooper, Sean Gunn, and Anthony Mackie. It will be released on May 4th, 2018.