Though Captain America: Civil War was great, a lot of people still consider the Russo Brother's best work on the MCU is The Winter Soldier. Apparently, Avengers 4 will be looking to end a storyline that was somehow jumpstarted way back in Captain America 2.
Joe Russo told Spin 1038:
"For us, this was the challenge of telling the story but also completing the personal journey that we started as storytellers in Winter Soldier. So the story that began in Winter Soldier and that ends in Avengers 4 is a, there's a very personal arc there for us. I think you'll understand our point of view as filmmakers and who we are as people when you watch all four of those movies together."
As of now, we still don't know what storyline they could possibly be thinking of ending, but there are several possibilities on what it could be. We could see the end of Bucky's Winter Soldier arc and see him turn into Captain America; we could see the eventual fall of HYDRA; or we could finally have Captain America's death.
The Russo's have teased that we should be afraid of the title for Avengers 4, and some are dead sure that we would be seeing the death of some major MCU players. A lot of people were expecting Cap to die by the end of Civil War, but we could be seeing that finally unfold onscreen with Chris Evans final MCU film, Avengers 4.
Avengers 4 is set to come out May 3, 2019, but catch Avengers: Infinity War April 27.