Marvel Studios revealed their calendar of (unannounced) films during the 2014 Comic Con marking their line-up of movies up to 2019. Here's what it looks like:
May 1, 2015 – The Avengers: Age of Ultron
July 17, 2015 – Ant-Man
May 6, 2016 – Captain America 3 (Untitled)
July 8, 2016 – Doctor Strange (Unannounced)
May 5, 2017 – Unannounced
July 28, 2017 - Unannounced
November 3, 2017 - Unannounced
May 4, 2018 – Unannounced
July 6, 2018 - Unannounced
November 2, 2018 - Unannounced
May 3, 2019 – Unannounced
While several rumours are hinting towards new superhero movies including Black Panther and Ms Marvel as the unannounced films, the date that particularly stands out is May 4, 2018 - which was announced separately a little later by the studio.
— Marvel Entertainment (@Marvel) July 23, 2014
"May 2018 is exactly three years after The Avengers: Age of Ultron will release, which is three years after the first Avengers so we wondered how that may affect when they were planning to release the third team-up," states a Screenrant report.
Marvel Studios president of production Kevin Feige said, "We haven't really talked about when we'll see Avengers 3. I presume we'd like to stick to three years between Avengers movies like we did...Oh, No, it was two years, wasn't it? What was it? Yeah, yeah. So it'd probably be three years about Avengers 2 and 3."
So, looks like there's a huge possibility that the May 4, 2018 slot might just get taken by Avengers 3 - which is also rumoured to have a different director in addition to several new members comprising the superhero team.
While the studio has given no indication on the future of Iron Man 4, reports suggest that the film is definitely happening and will most probably take up the May 3 2019 slot - that will also kick off Marvel's Phase 4 plan.