It might seem almost preposterous to consider a Fantastic Four team without one-half of it being Mr. Fantastic and the Invisible Woman, yet there have been instances when even Reed and Sue needed to exit the team for awhile--a gap that wasn't filled when Reed took it upon himself to remove the two of them in order to keep Sue out of harm's way when she became pregnant, but was addressed when the pair left the team after the events of Civil War to patch up their relationship. On that occasion, it was no less than royalty who filled in for them.
But much earlier, Steve Englehart, during his time as the book's writer, decided to shake up the team while Reed and Sue left to devote more time to their son, Franklin--moving up Ben Grimm, the Thing, to the leadership position, while returning to the fold former team member Crystal, who had proposed to fill in for Sue when she was sidelined during her pregnancy. That left a fourth position to be filled, by a virtual stranger to not only readers but also to the super-human community--an old friend of Ben's while he was on the wrestling circuit, someone who had gone through her own personal hell when she was held prisoner by Curtiss Jackson, known as the "Power Broker," and was so traumatized by the experience that she carries with her both rage against and fear of men. Unfortunately for the alchemist-villain known as Diablo, it's the "rage" part that she brings to the Fantastic Four's battle against him when showing up at the Baxter Building to visit Ben.
Though she's beaten Diablo to within an inch of his life, Sharon Ventura, the new Ms. Marvel, managed to save all four members of the original FF from certain death--and Ben believes that the place for her to recover from her past ordeal, while also bringing her focus and a sense of self, is the fourth slot on the team roster.