Fantastic Four is out. Despite the early positive feedbacks from the screening held at Atlanta, it failed. Internet hasn't been friendly since day 1. Nobody can say will it lead to the sequel set for 2017 or not. But here's a way I think Fox can still retain them.
1. X-Men Apocalypse. Apocalypse is said to change history as once said by Simon Kinberg in an interview with ING. So this implies that there will be a future glimpse(1). Fox can hire new cast for Fan4, sticking to comics this time. Trank's fan4 cast was realistic. People get adopted. But it didn't work for a comic book adaptation. Fox should keep in mind the requirements for the new cast. Taking no more chances.
Coming back to how Apocalypse wil help. It can done through an end credit scene future glimpse which is nothing but modern day. Where the four asks help from mutants to help them to save earth from Planet eating creature, Galactus.
2. This will lead to an X-Men Fantastic Four crossover along with Silver Surfer. If proper character development is done then people will love thenew fan4 cast and then Fox can do their origins in a prequel or simply not because it's done many a times. Fan4 can tell their origins to X-Men then a simple flashback scene will do it because how doesn't know about them?