Those frustrated that it took two years for The Crown to return for Season 3 will be happy to know The Crown Season 4 should have a much shorter turnaround. The only reason for the delay was that, as the story moved further into the '60s, it had to recast the characters to account for their aging. In lieu of Claire Foy and Matt Smith, Olivia Colman and Tobias Menzies now play Queen Elizabeth II and Prince Philip, and Princess Margaret, Antony Armstrong-Jones, et al have been swapped out, too.
According to Variety, there will likely be another cast change after Season 4 if the show sticks to the two-season pattern. In the meantime, though, the Season 3 actors will continue into Season 4, making the production timeline shorter. In fact, the next season has already begun filming, with some pictures surfacing of newcomer Emma Corrin as Princess Diana during her 1983 tour of Australia.
Season 3 doesn't introduce Diana, but the final scene is at the Queen's silver jubilee in 1977, which is also when a young Lady Diana Spencer met Prince Charles. As Diana told a reporter years later, "It was 1977, Charles came to stay. He was a friend of my sister Sarah's. [He came] for a [hunting] shoot. We sort of met in a plowed field."
Each season of The Crown spans roughly 10 years of the Queen's reign. Season 1 went from 1947 to 1955, Season 2 covered 1956 to 1964, and Season 3 did 1964 through 1977. Presumably, Season 4 will take viewers to around 1987, a few years after Prince Harry's birth but still in the middle of Diana's marriage to Charles and a decade before her untimely death. Using that timeline, Season 5 will end with her death, and Season 6 will end in around 2007. According to The Hollywood Reporter, the initial plan for The Crown was six seasons, but the first season came out just as Meghan Markle's relationship with Prince Harry went public. It's possible that Netflix will tack on an extra season to cover the more modern royal story.
As for when Season 4 will arrive, it will probably be in November 2020, and Princess Diana's romance with Prince Charles won't be the only thing it showcases. According to TV Line, X-Files actor Gillian Anderson has been cast as the first female prime minister Margaret Thatcher, who held the seat from 1979 to 1990. The need for a new prime minister became clear at the end of Season 4, with Harold Wilson announcing his resignation in the final episode. Other things the show could cover in the late '70s and '80s include major world events like the Cold War, Ronald Reagan's American presidency, and tensions surrounding the Berlin Wall.