The final season of 13 Reasons Why doesn't start up immediately after the events of the last season. There wasn't a suspenseful cliffhanger that raised questions the show immediately needs to answer. But some time has passed since 13 Reasons Why season 3. Consider this your orientation for the last new season.
Luckily, the festive season can provide a simple answer: One month has passed between seasons. Season 3 ends with the main characters celebrating Thanksgiving, and the events of season 4 begin at Christmas. In fact, the season 4 premiere is titled "Winter Break," so it's super clear when this is taking place. In that brief hiatus, Justin Foley (Brandon Flynn) has completed a short rehab program for drug addiction.
That said, season 4 opens with a flash forward to a funeral six months in the future — which would be June, presumably around the time most of our main characters have graduated from Liberty High.
The 13 Reasons Why timeline has not been consistent over the four seasons of the Netflix series, which sometimes makes it difficult to track how much time has passed. (Not to mention the fact that season 1 aired in 2017, and a lot of the cast members are starting look like 20-somethings — which they all are.)
In story time, only fourteen months have passed since Hannah Baker (Katherine Langford) died by suicide, the inciting incident of the series. Each season has roughly covered one semester of high school in the lives of the teenage characters. But the actual "present day" events of each season range from weeks to months, and each season uses flashbacks and flash forwards in different ways.
Most of the characters are in Clay Jensen's (Dylan Minnette) and Jessica's (Alisha Boe) graduating class, and so for them, seasons 1 and 2 were junior year while and seasons 3 and 4 were senior year. That also explains why some characters, like Marcus (Steven Silver), Ryan (Tommy Dorfman), and Courtney (Michelle Selene Ang), left 13 Reasons Why as regulars after season 2. They graduated, and hopefully got very far away from Crestmont.
Since it is final year, that means that the characters on 13 Reasons Why are applying to college and exploring options for their futures, as well as getting ready to graduate. They're reflecting on the traumatic events of the past two years while trying to keep their secrets from catching up with them.
This is a challenging time for all high school students, and much more so for these Liberty High kids who have been through so much in such a short period.