Dom’s wife and partner in crime, Letty was killed at the beginning of the fourth film, “Fast & Furious” (2009), after she ran afoul of a master criminal. She saw the error of her ways at the end of that movie, and she’s been back with Dom and company ever since.
How come Han is alive in Fast 4?
He was also killed in that film, dying in a fiery car crash in the Tokyo streets. But he was not gone forever—the fourth, fifth, and sixth films revealed themselves to be prequels to Tokyo Drift, and therefore able to feature a still-breathing Han.