Iron Man was one of the most important characters in Infinity War as after Thor and Thanos, he is the one who got the most amount of Screen time in the film. Infinity War left him alive so he could uncover a very crucial plot in Avengers 4. Doctor Strange saw that he was really important in the future which actually had the Avengers beat Thanos and that is why he bargained for Tony’s life in exchange for an Infinity Stone.
Tony wearing a SHIELD outfit which does suggest that in the absence of Nick Fury and the reformation of SHIELD, he has now become in charge himself.
Since we see a difference in his look, it also seems that there has been a sort of Time Jump after the big snap took place. This coincides with what Gwyneth Paltrow mentioned in an interview saying that Pepper was indeed pregnant and with the time jump, Tony has been a father for a while, and with half of the world now gone, it is upon him to actually take charge and protect the leftover people from the threat that still exists, Thanos.
In the very first concept arts that we saw for Avengers Infinity War last year when the movie began production, there were just 4 of them shown to us and out of them, 3 were more or less used. The Thor and Rocket team up was teased back then and it came to being in Infinity War.
But there is the one which involves Tony and multiple Iron Man suits that he keeps building which did not necessarily get used in the movie. As we know that Tony has taken on this role of becoming the savior of Earth for a while now, which is why it is just assumed that this Concept Art is just to portray that and was never going to be used in the film.
But the fact that we are actually going to see Tony take on a more hands-on role as the protector of Earth from now on, it does feel that this Concept Art would come into play in Avengers 4. Theories say that Tony would have returned to Earth from Titan and seeing all the death and destruction, he will continue the path he started in Avengers: Age of Ultron.
These Iron Man suits that we see are probably more advanced Iron Legion soldiers. Since the policing resources of the entire world will also have been reduced to half, Tony will probably put an army of these out there. He is also said to be the new leader of SHIELD so this is probably how he uses his resources along with SHIELD’s to protect everyone.
The other theory is a continuation of Tony being a father. Pepper has always wanted Tony to leave the world of crime fighting and come home which is why she also denied being pregnant in Infinity War. After Infinity War, in order to make Pepper happy, Tony could make these Iron Legion drones so he would not have to put himself in harm’s way.
Well, we do know that even if Tony is a father in Avengers 4, he will still have to fight the final battle against Thanos and since he has the most important role in the movie, he could not be pushed out of action just like that. What would actually be great is that if Tony brings an entire fleet of these Bots at the final battle of Avengers 4, just like he did in Iron Man 3. But this time, the other Avengers will be around as well.
Also, there are rumors that suggest a Thanos Buster armor could be in play, and that would be the only way Tony tops the suit he wore in Infinity War. The Nanotech suit was next level stuff and we don’t know how Tony will actually top that.
We will only find out about that when Avengers 4 opens on May 3, 2019.