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

Prepare yourself, because "Slow Horses" is set to hit the small screen in just over two months. Yes, this means we don't have long 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 we 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. 

If that wasn't enough excitement, "Slow Horses" helped break the Apple TV Plus record with 9 Emmy nominations, including outstanding lead actor for Gary Oldman. That's right, this is now an award-winning show, and it deserves that recognition above any other Apple original. 

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

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

As of June 27, we finally have a release date for "Slow Horses" season 4 thanks to Apple TV Plus. 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, and he was right about that. 

"Slow Horses" season 4 will arrive on the streamer on September 4. This is earlier than we expected, but of course, no one is complaining. Apple TV Plus announced the news on Twitter (X) with a bunch of first look photos. In just over two months, we'll see the gang back in action again. 

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

'Slow Horses' season 4 cast

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.