Since basically the entire Marvel superhero roster was wiped out by the snap of Thanos' finger in Avengers: Infinity War, fans have been coming up with theories as to how some of their favorite heroes may be revived in Avengers 4.
A theory posted by Reddit user commander_wong shared a theory where a hero who died in Infinity War has already come back to life, and that's Captain America.
Commander_wong's post read, "Thanos ended his short battle with Cap with a punch to the back of the head. Many wondered how Cap recovered in mere seconds from a punch that even took out the Hulk and the most logical explanation is that he didn't As Thanos used the Time Stone to bring Vision back, the surroundings also reverted: Natasha was no longer trapped under pillars of stone, Bruce was no longer fused to a rock, and Cap showed no signs of injuries from a blow that should've easily killed him."
As Comic Book states, if this theory is true, it would explain how Captain America appeared again at the end of the movie.
Regardless of how Captain America got brought back to life (or if he never truly died), he survives with half of the universe against Thanos' snapping, as we'll see in Avengers 4.