Like the comic book Thanos, Josh Brolin's iteration of the Mad Titan has had only one goal in the Marvel Cinematic Universe: to obtain the six Infinity Stones. Each of those Stones is powerful on their own, but when all six are inserted into the Infinity Gauntlet, Thanos will be able to twist reality as he sees fit. Avengers: Infinity War will see Thanos trying to collect all these artifacts for himself, but according to Brolin, he may not succeed in obtaining all of them by the time the movie is over. When asked if Thanos will put all six Stones into the Gauntlet in Infinity War, Brolin said:
There's a few in there, and he's always pining to get another one in there. So ultimately we don't know. I haven't done the second movie, so I don't know.
Since Marvel hasn't revealed any details about Avengers 4 other than confirming some of the established MCU characters who will return, fans have been speculating for a while about exactly how closely that movie will tie into Avengers: Infinity War. Co-director Joe Russo said last year that there would be some "cross pollination" between the two moves, but Josh Brolin's comment suggests that we'll see the Thanos story arc continue in Avengers 4 if Infinity War ends with him not having obtained all the Infinity Stones. Brolin also clarified to Good Morning America that he not only has he not seen the Avengers 4 script yet, he doesn't even know the script is finished right now.
So far five of the six Infinity Stones are accounted for in the MCU, with the Soul Stone as the only one that hasn't been officially introduced yet. The Space Stone (a.k.a. the Tesseract) is locked up in Asgard's vault, although with Thor: Ragnarok coming up in November, who knows if it will remain safe there. The Power Stone (a.k.a. the Orb) is under the protection of the Nova Corps. The Reality Stone (a.k.a. the Aether) was given to The Collector in Thor: The Dark World, although since his museum was destroyed in Guardians of the Galaxy, it's unclear if he still has it. Finally, both the Mind Stone and Time Stone (a.k.a. the Eye of Agamotto) both rest on Earth, with the former powering The Vision and the latter resting in Kamar-Taj.
Holding on to just one of these Stones imbues its holder with amazing abilities, but Thanos won't settle for anything less than all of them, even if it takes him another movie to make that happen. With the help of the Black Order, Thanos will lay siege to Earth and the entire universe in Avengers: Infinity War, and only the Avengers, the Guardians of the Galaxy and select other superheroes stand a remote chance of stopping him, though they may not ultimately succeed in doing that until Avengers 4.
Avengers: Infinity War comes out on May 4, 2018, and Avengers 4 will follow on May 3, 2019. While we wait for more news about either of these movies, be sure to read our detailed synopsis of the special Infinity War footage that screened at D23 this past weekend.