The Russo Bros. shared an image featuring a glove with four fingers extended.

The trailer for Avengers: Infinity War isn't even out yet (at least, not officially), but the Russo Bros. are already on to the next installment.
The directors on Thursday posted a teaser image marking the begin of filming for Avengers 4, and appropriately, it's a glove showing off four digits. (Remind anyone of Hugh Jackman's three-clawed tease for what became Logan?). No word on who the glove belongs to, but the Russo's Facebook page has plenty of entertaining theories from fans, including commenters speculating it belongs to Captain America or time-traveling villain Kang the Conqueror.
Marvel has not revealed the title of Avengers 4 yet because the title itself is a spoiler, and little is known of the company's plans following the movie. Only a 2019 Spider-Man: Homecoming sequel, due out two months after Avengers 4, and an undated Guardians of the Galaxy Vol. 3 are on the docket officially. Guardians director James Gunn has said he and Marvel Studios chief Kevin Feige are tossing around ideas for the future of Marvel's cosmic movies.
Feige told Collider in April that 2018's Infinity War is the culmination of 10 years of storytelling, and that what comes next may be something different.
"Certainly as we get to Infinity War, there is a sense of a climax, if not a conclusion to, by the time were at untitled Avengers 4, the 22 movies that will have encompassed the first three phases of the MCU [Marvel Cinematic Universe]," Feige said. "And what happens after that will be very different. I dont know if its Phase 4, it might be a new thing.
Avengers: Infinity War is set to open May 4, 2018, while Avengers 4 will hit theaters May 3, 2019. Here's the Russos' tease: