Who do you think will have the most pivotal role in defeating Thanos in Avengers 4?
I agree with the other answers here. It seems fairly obvious that Captain Marvel and Iron Man will play pivotal roles in Avengers 4. We can also guess that Stormbreaker-wielding Thor will also have an important role.
But to me, the one that's been subtly hinted at the most is Bruce/Hulk. His hero's journey takes place across multiple movies, starting at Age of Ultron as the first threshold, then Ragnarok as the challenges/temptations, then Infinity War with the death/refusal, which leads to Avengers 4 for the return/rebirth.
Captain Marvel is going to appear as backup, and she's been confirmed to be the most powerful superhero the MCU has ever seen. That's cool and all, I just hope she won't be a simple deus ex machina. That will likely not be the case, though, because Marvel has shown that they're smarter than that and since Carol Danvers (and Ant-Man, too) have ties with the quantam realm, that's where I'm assuming their usefulness will really kick in.
Tony Stark is also going to be instrumental in defeating Thanos, otherwise Doctor Strange would not have asked Thanos to spare his life in exchange for the Time Stone. Exactly what it is Tony will do remains to be seen.
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Doctor Strange is sworn to protect the Time Stone by a millennia-long oath taken by the wizards of Kamar-Taj. He chose to break that oath to save Tony Stark after seeing the future. Tony Stark is so important to defeating Thanos and returning things to normal that Strange allowed Thanos to complete the Infinity Gauntlet to save him.
If it's not Stark, I'll go with Thor or Carol Danvers. They're the most powerful remaining heroes. Scott Lang is also a possibility, since he is currently the closest one to having time travel capabilities.
Most likely Iron Man and Captain Marvel. We already know that Captain Marvel will be incredibly important as the MCU moves on, and that she's the most powerful of all the Avengers.
Iron Man, on the other hand, has a distinct connection to Thanos and was the only one besides Thor to injure Thanos even a little bit. Plus, he's sort of the main character of the MCU and it's his last movie as far as we know, it just makes sense.
I suppose Thor may have a big role too since he has a big personal beef with the Mad Titan. Of course, only time will tell which of these heroes get the last blow.