Slow Horses season 4 trailer gives big insight into new Head Dog Emma Flyte as she handcuffs Jackson Lamb

The Slow Horses season 4 trailer has finally arrived and it gives viewers a fascinating insight into Emma Flyte, a major new character in the Slow Horses world.

Emma, played by Ted Lasso 's Ruth Bradley, is a huge character in the books by Mick Herron which the Apple TV Plus hit is based on. And she comes in at the start of the new series, which again stars Gary Oldman as slob spy boss Jackson Lamb.

Ex-cop Emma is now the Head Dog, in charge of MI5's internal police squad, working for top spook Diana Taverner (Kristin Scott Thomas). But is Diana telling her the whole truth about a major bomb blast that's rocked England? One thing is clear: Emma is not taking any nonsense from Lamb.

The trailer, below, sees a couple of wonderful exchanges between Flyte and Lamb. Emma: "I heard you're in charge of the rejects." Lamb: "They don't like being called that". Emma: "What do you call them?" Lamb: "The rejects."

In a further exchange, Flyte is seen putting handcuffs on Lamb. Lamb: "Handcuffs won't be necessary, come on."

Emma: "I'd rather not take any chances with a man who looks like he gropes people on buses."

Lamb: "You’re being hurtful about my appearance. I might have to call HR".

The trailer also features a huge explosion. The makers tease "a bombing that detonates personal secrets, rocking the already unstable foundations of Slough House".

Emma Flyte becomes embroiled in Jackson Lamb's world as she has to work out whether the Slow Horses are on the right path and also exactly what is motivating her boss Diana Taverner. The plot heavily involves River's grandad David Cartwright, who used to work with Lamb back during the Cold War.

Emma isn't the only new character in the new series. Joanna Scanlan joins the show as fresh Slough House arrival Moira Tregorian where she meets reject spies River Cartwright (Jack Lowden), Louisa Guy (Rosalind Eleazar), Roddy Ho (Christopher Chung), Shirley Dander (Aimee-Ffion Edwards) and Marcus Longridge (Kadiff Kirwan).

Slow Horses season 4 is premiering on Wednesday, September 4, on Apple TV Plus.

The first two episodes will be available to watch immediately, with a single new episode then releasing every Wednesday until October 2.