'Slow Horses' season 4: Everything we know so far

Gary Oldman in Slow Horses
(Image credit: Apple)

"Slow Horses" season 4 is currently in production, and we shouldn't have to wait too long to see it. Filming started in the spring of 2023 and has since finished, but there may be more time to wait until we see the Slow Horses navigate their next big case. The show has been a critical hit for Apple TV Plus and famously, each season ends with a trailer for the following season. 

Season 3 was no different. The third season of this British spy thriller was incredible — it was the best TV show I watched in 2023 — and, as is tradition, we got a trailer for season 4 at the end of the sixth and final episode of the season. 

Because of this trailer, we already know a fair amount about the upcoming season of the show. And we know that we'll see a trailer for season 5 after the final episode of season 4 since Apple already greenlit a fifth season

So here's everything we know about "Slow Horses" season 4, including the show's potential release date, casting updates, trailers, and more.

'Slow Horses' season 4 release date and where to watch

Despite having a trailer for season 4, we don't quite know when the next season of "Slow Horses" will arrive on Apple TV Plus. But showrunner Will Smith (no, not that Will Smith) said in an interview with Decider that the first episode will definitely arrive on the streaming service before the end of 2024. 

We expect "Slow Horses" to be released on Apple TV Plus at the end of November 2024. The last season arrived on Wednesday, November 29, 2023 and the season before that arrived on Friday, December 2, 2022. If I had to guess an exact date for season 4, I'd go with Wednesday, November 27, 2024, with the first two episodes dropping on that day and the season 4 finale hitting Apple TV Plus on Wednesday, December 25. Talk about an incredible Christmas present.

An image indicating spoilers are ahead.

(Image credit: Future)

Spoiler alert: Spoilers for seasons 1-4 of "Slow Horses" beyond this point

'Slow Horses' season 4 cast

(L to R) Kristin Scott Thomas as Diana Taverner and Gary Oldman as Jackson Lamb in Slow Horses, one of the best Apple TV Plus shows

(Image credit: Apple TV+)

Despite a now familiar ensemble cast, the show's main cast is a surprisingly small group. Gary Oldman's Jackson Lamb, Jack Lowden's River Cartwright and Kristin Scott Thomas's Diana Taverner are the only core members of the "Slow Horses" cast.

However, thanks to the new trailer and some casting announcements we can make an educated guess at who to expect in season 4. Here's the full cast we know so far:

  • Gary Oldman as Jackson Lamb, the washed-up spy in charge of Slough House, aka the "Slow Horses"
  • Kristin Scott Thomas as Diana Taverner, the former Deputy Director and now (presumed) head of MI5 who has a long history with Lamb.
  • Jack Lowden as River Cartwright, one of Lamb's "Slow Horses" who is a relatively competent MI5 agent and was raised by his grandfather who was a former MI5 legend.
  • Saskia Reeves as Catherine Standish, Lamb's secretary and a recovering alcoholic who seemingly leaves Slough House at the end of season 3.
  • Rosalind Eleazar as Louisa Guy, one of Lamb's "Slow Horses" who is dealing with the trauma of a past relationship with a coworker.
  • Christopher Chung as Roddy Ho, a member of Slough House who is a tech expert and incredibly annoying.
  • Sophie Okonedo as Ingrid Tearney, dishonorable ex-head of MI5 who could appear in season 4
  • Samuel West as Peter Judd, Conservative MP who could also appear in season 4
  • Aimee-Ffion Edwards as Shirley Dander, one of Lamb's "Slow Horses" who has anger issues and a bit of a drug habit.
  • Kadiff Kirwan as Marcus Longridge, one of Lamb's "Slow Horses" who has a serious gambling problem.
  • Jonathan Pryce as David Cartwright, River's grandfather who was an MI5 legend but is slowly losing his mind.
  • Hugo Weaving as Frank Harkness, likely season 4's main antagonist.
  • Ruth Bradley as Emma Flyte, seemingly a newly introduced member of MI5.
  • James Callis as Claude Whelan, seemingly Taverner's new second-in-command at MI5.
  • Joanna Scanlan as Moira Tregorian, no character details revealed yet
  • Tom Brooke as J.K. Coe, no character details revealed yet

'Slow Horses' season 4 trailers

Apple has yet to drop an official trailer on YouTube, but thankfully some intrepid soul has uploaded it for us. You can watch it below, though from here on out we get into spoiler territory.

The first trailer for season 4, which we see at the end of season 3, shows Jackson Lamb in a miserable state — not that he'd tell you. Meanwhile, a bomb has exploded in London and MI5 is in shambles. Emma Flyte (Ruth Bradley), appears as a new member of MI5 and is now seemingly a liaison between Slough House and MI5. MI5 is now led by Diana Taverner, and she's using Flyte to inquire about the state of Lamb's "rejects."

And then there's River Cartwright, who unsurprisingly is now stuck in another dangerous situation opposite a menacing Frank Harkness (Hugo Weaving) who either wants River to join him or die. As if that wasn't bad enough, he's also dealing with his grandfather's (Jonathan Pryce) deteriorating health.

If this trailer for season 4 is any indication, we can expect the same wonderful blend of tension, action, wit and charm that makes "Slow Horses" so great.

'Slow Horses' season 4 plot

Season 4 of "Slow Horses" has been confirmed to be an adaptation of Mick Herron's 2017 novel "Spook Street," the fourth novel in his Slough House series. I'm not going to go into full spoilers here — if you want to know everything that could happen, do some spy work and read the book for yourself.

I will, however, give you a brief synopsis of what to expect, and that will entail some spoilers from the past three seasons as well as potential spoilers for season 4.

In season 3, we start to see Jonathan Pyrce's David Cartwright (River's grandfather) start to show concerning, early signs of dementia. Those concerns, unfortunately, are real, and in "Spook Street" River must deal with his grandfather's deteriorating mental state while also dealing with a bombing that has killed dozens of civilians. Season 4 will certainly be messy and won't pull any punches. It'll also be the last time we see a certain cast member — at the end of the season 4 teaser trailer Gary Oldman's Jackson Lamb announces one of his team has died.

You can stream season 3 again on Apple TV Plus

Malcolm McMillan
Senior Streaming Writer

Malcolm McMillan is a senior writer for Tom's Guide, covering all the latest in streaming TV shows and movies. That means news, analysis, recommendations, reviews and more for just about anything you can watch, including sports! If it can be seen on a screen, he can write about it. Previously, Malcolm had been a staff writer for Tom's Guide for over a year, with a focus on artificial intelligence (AI), A/V tech and VR headsets.

Before writing for Tom's Guide, Malcolm worked as a fantasy football analyst writing for several sites and also had a brief stint working for Microsoft selling laptops, Xbox products and even the ill-fated Windows phone. He is passionate about video games and sports, though both cause him to yell at the TV frequently. He proudly sports many tattoos, including an Arsenal tattoo, in honor of the team that causes him to yell at the TV the most.

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