Tony Stark/Iron Man Will Sacrifice Himself
We all had our assumptions that Tony would die in Infinity War. And we were wrong! But that doesn't mean he'll make it out of Avengers 4 alive. Doctor Strange surrenders the Time Stone in exchange for Tony's safety after seeing all the ways they can lose to Thanos, which means that Tony's survival, at that point, is key in Thanos's defeat. There are several reasons and ways that Tony could end up dying, but the most realistic is that Tony will sacrifice his life for his teammates or the entire half of the universe that was wiped away. Tony's death would have the most impact as he is the first Avenger we met, he's very likely expecting a child with Pepper soon, and he is the one person who's gone through the most in the MCU development-wise. Infinity War shows a Tony determined to enjoy his life, only for his friends to get ripped away. We can already imagine falling to pieces again in a scene similar to Peter's death, but in reverse.