I think it's going to be Cap that gets the permanent death.
Tony already "sacrificed" himself in the 1st Avengers. He & Rogers had the mindset that going into the worm hole was a one way ticket.
Tony also had the moment in IW that for a split second, everyone thought he was going to die.
Cap hasn't had those types of moments yet in any Avengers movie. He would be the 1st person to volunteer to sacrifice his life to save the world, he just hasn't gotten that opportunity yet. (His 1st Solo movie doesn't count in my opinion.)
He will get that opportunity in A4 & he will take it without hesitation.
Tony has never been serious about retirement in the past, he only has done it for others, not because he wanted to. After everything in IW, & if the rumors of him having a kid in A4 are going to happen, then I see him retiring for real this time because he finally is DONE risking his life. Like someone said above, Thanos is his story arc. So ending the thanos threat will bring that to a close.
I can for sure see him being a mentor in future films, providing the tech for everyone like he does now. But he will be done fighting.
Also, RDJ loves the character, loves the fans, so I can see him being fine with being in cameos till he is old & grey. It would be REALLY dumb for them to kill him off. I 100% don't believe they will.
I'm am curious though if they take a bit from the comics with Nebula still being alive. I think it would be dumb to have a human use the Gauntlet with all the stones. Even once. Even a little bit.
It would take away from the fact of how powerful all 6 stones are and WHAT they are & how powerful Thanos truly is.
But like in the comics, Nebula can use the gauntlet, not for very long though. But enough to do something meaningful. I can see her dying in A4 as well as Cap. Unless they plan on expanding her story with Gamora in the next Guardians movie.
I am probably in the minority, but I REALLY REALLY hope they don't kill off Thanos. I hope that is one villain they just de-power, & let him go live on his farm lol. He really is my favorite.
I know they will kill him off & that is one part of the movie that I am NOT looking forward to. At all. Nothing can convince me otherwise. THANOS DID NOTHING WRONG.
He was my favorite in IW.