This Is Us season 4's fall finale confirms that Kate (Chrissy Metz) and Toby (Chris Sullivan) are heading for a legal separation. The fan-favorite family drama has wrapped up the first half of season 4 fairly early with a capper full of time-jumps and plot surprises. One specific narrative point, however, has long been teased as baby Jack's parents face a difficult path ahead of them.
Kate and Toby's married life this season primarily focused on their shared concern over their son. While things seemed to be going well in his first few weeks, their relationship has had one too many small arguments in season 4 that it's started to build up to something serious. Most of their issues stemmed on the pair going separate directions when it comes to their respective weight loss journeys. Toby decided to take his health seriously as he devoted a lot of time in the gym, while Kate has started overeating - an unconscious habit she's had to cope with stress.
While the couple tried to get past their initial disagreement once Kate found out that Toby had been sneaking behind her back to work out, the issue continues to pop back into their lives. Each time, the argument is slightly different, but at its core is their still unresolved issue about Kate feeling betrayed by Toby's actions - she admitted herself during the fall finale to her sister-in-law, Beth (Susann Kelechi Watson). As they continue to coast through a bigger problem by trying to act like everything’s fine, a subtle detail from This Is Us' latest episode revealed that the pair are headed for divorce.
The timeline is all over the place for the episode titled "So Long Marianne," but in the present day, the Pearsons celebrate Thanksgiving at Randall's new Philadelphia house. On their way to the East coast, the couple already had a mini-argument on the plane as Kate came clean about baby Jack's real solid food experience. As they do, they tried to brush past the issue, but Kate's irritation on her husband only worsened as Toby obnoxiously blabbed about his gym escapades. Things became more serious when Kate discovered not only was Toby ranting about her on a CrossFit text group, but that a female member replied to him with a rather flirty message about being there for him if he needs anyone.
There wasn't any big argument after that, possibly because Kate was saving it for later, and due to the This Is Us outing ending with a nine-month time-jump where Rebecca’s (Mandy Moore) Alzheimer’s has gotten so much worse that she had to be brought back home by the police after having a panic attack. While signing papers for her mother’s safe return, we see Kate’s name back to Kate Pearson (even signing with it) rather than Kate Damon. This means that sometime in between the November 2019 and the next nine months, the two will be legally separating resulting in Kate going back to her maiden name.
This coincides with a couple of insinuations we've seen from This Is Us flash forwards that Kate and Toby's relationship tread a dark path. We've seen a depressed Toby in the future without his wedding ring, and as he joined the Pearsons to visit the dying Rebecca, there's no mention or appearance from Kate.