As we understand it though from multiple sources, Fox has not given Marvel back the Fantastic Four. At least not yet, but we do expect that the Fox-Marvel partnership and Fox programming announcements appearing on Marvel.com yesterday to be just the beginning.
Fox finds itself in the same position Sony Pictures was in last year. Once the infamous Sony hack confirmed that there were discussions between Sony and Marvel about co-producing movies and bringing Spider-Man into the Marvel Cinematic Universe, Sony was in a losing battle. It was either make The Amazing Spider-Man 3 and Sinister Six continuing off the lackluster Amazing Spider-Man 2 in an increasingly competitive environment against larger comic book universes from DC Entertainment and Marvel Studios or make a Spider-Man movie where the character could interact with The Avengers and the happenings of the MCU. Which would make more money and which is more exciting for moviegoers? Spider-Man shouldn't exist in a world where he's the only hero. Sony and everyone involved with The Amazing Spider-Man agrees.
For Fox, they can make little money (or none at all) with another Fantastic Four movie where the heroes are in a world on their own, fighting a limited library of villains, or these characters can exist as they do in Marvel Comics' greatest stories where Reed Richards bounces ideas off of Bruce Banner, T'Challa, and Tony Stark; where The Avengers can fight Doctor Doom, and where the Guardians of the Galaxy can team-up with Silver Surfer. Expectations for the future of Fox's Fantastic Four have to compete against the idea of the FF in the MCU so continuing with them on their own isn't an appealing option.
Again, keep in mind, insiders from three sources (including Fox in a statement to comicbook.com) claim that the Fantastic Four are not back at Marvel. Such a deal can be made though. After all, with producers Simon Kinberg and Lauren Shuler Donner involved in the Marvel-Fox TV partnership and exec producing their two new shows, they're the ones who can work something out with Twentieth Century Fox and Marvel Studios boss Kevin Feige. Don't expect to see a Fantastic Four announcement for 2020 though. It's not part of Marvel's plan, but Phase 3 can shift again (it already has twice) when more deals are made. And there will be more deals made.
The question though is whether or not Marvel Studios even wants the Fantastic Four. We expect not as a standalone feature, but their characters can certainly be used in other movies, especially their villains and cosmic characters.
For Fox, if their X-Men franchise starts to go cosmic after X-Men: Apocalypse ends the universe (and sort of relaunches it) as we currently know it, they can't give up the Fantastic Four because they need their vast library of non-Earth lore for future movies.
Sources: Fox, Marvel, Comicbook.com