Spoiler Alert: Avengers Infinity War Spoilers below! These are major spoilers and avoid if you haven’t seen the movie!
Thanos succeeded in his mission well before Avengers: Infinity War ended. He put together all the six Infinity Stones, created his Infinity Gauntlet and did the finger snap which wiped out half of the life in the Universe. Now in Avengers 4, the most crucial hero left might be Quicksilver!
Assuming Avengers 4 is called Avengers: Forever, it might be based on the comic book story bearing the same name which featured heroes from various timelines getting removed from their timeline and placed in the present to come together and battle a common enemy. Since half of the Avengers are already dead or wiped out from the visible existence, it appears obvious that some of them will be revived somehow. If the survivors manage to find a time to go grab Quicksilver and bring him to the modern day, he would definitely be very useful.
Just think about it! Is there anyone else fast enough to snatch the Infinity Gauntlet or the Infinity Stones from the Mad Titan without getting caught by him? Quicksilver will give the team in space, enough time to tear off the Gauntlet from Thanos’ grip before the outburst from Star-Lord took place and resulted in the annihilation of trillions.
In certain scenarios, Quicksilver also has the capability of time travel using his extreme speed. If he is revived by some means, he could become the one who would connect the past heroes with the present and make them come together against Thanos. If not this then the only other option would be to recover the Time Stone, but, the Avengers have no idea where Thanos is so it would be a much tougher mission to start with. In any case, whether they discover an alternate method of time travel or they get hold of the Time Stone, either way, they have every reason to revive Quicksilver.