I guess the most likely scenario is that it under-performs at the box office and/or gets critically mauled, and Fox either sell the rights back to Marvel or they reach a shared ownership agreement like they did with Sony. But I don't think they're gonna do that lightly.
Apparently the relationship between Fox and Marvel has broken down over the years to such an extent that they don't actually communicate that much anymore. Though to be fair no studio in town has that great a relationship with Fox
I just don't see a situation where Fox would want to share X-Men to the point that they would happily cut loose the F4 so that they can continue to do as they please with X-Men.
Besides, a brightly coloured, 'faithful' F4 adaptation would probably fit better in the MCU than what Fox is doing with their Marvel IPs.