What do we know about the plot of Season 4?
Season 3 ended with June successfully transporting more than 50 children out of Gilead and to safety in Canada. In the process, however, she ends up getting shot in the shoulder, left injured and stuck in Gilead. Despite this cliffhanger, it's safe to assume that she'll be fine: The Handmaid's Tale is, at its core, June's story. "When June dies, our guide dies, and our show dies, showrunner Bruce Miller told Vanity Fair.
We'll also likely see more on the Canadian side this season. Miller told TVGuide that "the people in Toronto are as much a part of the story as the people in Gilead." Season 4 will likely see Moira and Luke dealing with the realization that June was the one who orchestrated the children's escape. Of course, there's also the matter of dealing with the Waterfords, who are set to go on trial in Canada and potentially be held accountable for their actions under the Gilead regime.
Finally, it looks like Season 4 will also give Nick more time in the spotlight. Miller told TVGuide that they had planned to develop Nick's role more during the third season, but were unable to do so due to time constraints. Now, it seems that we'll be learning more about him and his relationship with June in Season 4.
Season 4 may draw some inspiration from Atwood's recent Handmaid's Tale sequel, The Testaments, but given that the book was only released in September 2019, it'd be a relatively quick turnaround for Season 4 writers. Atwood has read scripts for the series with a mind towards the sequel however, so we can definitely expect some interplay between the sequel and the show in the future (although Hulu has already picked up the rights for a separate adaptation of The Testaments).