In reference to the rumor, reported Marvel insider Roger Wardell wrote on Twitter, "Fantastic Four rumor that's going around is totally false, [Marvel Studios has] enough on their plate already."
Fantastic Four rumor that's going around is totally false, they have enough on their plate already— Roger Wardell (@RogerWardell) June 6, 2019
Although Wardell's real identity is unknown, he's been a reliable source of information in the past, having revealed key details about Avengers: Endgame before the film's first trailer released. He's also provided information about Guardians of the Galaxy Vol. 3 and Doctor Strange 2 that may prove true or false when those movies release.
First appearing in 1961's Fantastic Four #1, the team consisting of Reed Richards/Mister Fantastic, Sue Storm/Invisible Woman, Ben Grimm/The Thing and Johnny Storm/Human Torch was created by Stan Lee and Jack Kirby. The team is one of the most recognizable in comics, and there have been several movies and TV shows based on it. Ant-Man director Peyton Reed was rumored to be pitching Marvel Studios a Fantastic Four movie set in the 1960s involving the Quantum Realm and, potentially, Black Knight.
There was a lot working against this rumor in the first place, including the fact Reed previously described an idea for a Fantastic Four film set in the '60s that he'd pitched to Fox. Additionally, Marvel Studios President Kevin Feige said earlier this year the studio plans films five years in advance. This would have meant Marvel Studios was planning for the Fantastic Four reboot before acquiring the rights to the characters. Although that's possible, the studio doesn't seem to work that way, as evidenced by Feige's confirmation Marvel Studios was working on its five-year plan before the acquisition was set, which he used to explain why the X-Men wouldn't appear for "a very long time."
At this time it's unknown when and how the Fantastic Four will join the Marvel Cinematic Universe.