Watch them in the order they were released. The Avengers (2012) has to introduce six people who do not yet work as a group. Age of Ultron (2015) only makes sense once that group exists. Infinity War (2018) spends its running time on an opponent the earlier films have been pointing at, and Endgame (2019) is the answer to what Infinity War leaves behind. There is no separate chronological cut of these four: the story on screen is already in time order. Together they run 614 minutes, which is one long day or two ordinary evenings. The first two were directed by Joss Whedon; the last two by Anthony and Joe Russo. If you only have time for the pair that closes the story, start at Infinity War — Endgame assumes you have just seen it.