Unforgotten series 4 ending explained: Who killed Matthew Walsh?

With Cassie gone, Sunny was forced to continue the historic murder investigation into the death of Matthew Walsh, whose headless and handless remains had been found, frozen, at the start of the season.

The four Unforgotten suspects – Fiona Grayson, Elizabeth “Liz” Baildon, Dean Barton, and Ram Sidhu – were once trainee coppers in the 1990s.

They were all present the night Matthew Walsh died in an allotment; and they all colluded to hide the body. But which one killed him?

Who was the murderer in Unforgotten series 4?

Matthew Walsh died in the 1990s after he tripped in an allotment, knocking himself out - before the murderer found Walsh, and pushed a metal foundation pen nib through his head wound.

Several of the suspects had had direct contact with Walsh a few weeks prior to his disappearance: he had sexually assaulted Fiona, and had beaten up Ram. Liz had an on-off relationship with Fiona, and could well have murdered Walsh as a means of avenging her girlfriend.

However, it turned out that another suspect had a connection to Walsh. Dean (the dodgy businessman who had been making mysterious trips to Calais) had changed his own name from Quinn to Barton, in order to disguise his connection to his career criminal family.

His supportive older brother, Stephen, had been more encouraging of Dean's ambitions beyond crime, even gifting him an engraved fountain pen. However, he was murdered by a rival gang member: none other than Matthew Walsh.

And it was Dean who killed Walsh, as a means of avenging his brother.

Sunny made the connection after a previous tip-off from Cassie, when she suggested looking into how Dean's brother had died.

The theory came together when the team tracked down the pen's maker, who looked into the purchase records and found that it had been bought by Stephen and engraved with Dean's initials, as a birthday gift.

Dean had always seemed the most good-natured and personable out of the four suspects - he had proved a “lovely” interviewee when Cassie spoke to him during episode three.

However, as he told her at the time, you never really know what someone is capable of…

• For the latest news and expert tips on getting the best deals this year, take a look at our Black Friday 2021 and Cyber Monday 2021 guides.

Will the other Unforgotten suspects go to prison?

Expectant father and serving copper Ram will likely face prison time, as not only was he charged with preventing a lawful burial of a body (as were Liz and Fiona), but he had also been involved in Dean's cocaine smuggling operation.

Bank records revealed that Dean had been paying him large amounts of cash in exchange for bribing a customs officer (who had also been arrested, and proved very cooperative).

Meanwhile Fiona is facing the additional charge of practising as a therapist with a fake licence (she had never applied for a real one as she wanted to conceal her historic dangerous driving conviction). However, despite everything, her husband has forgiven her.

Finally, Liz has had to resign from the police force after serving nearly 30 years (a similar length of time to Cassie), and may also face prison time for her part in hiding Matthew Walsh's body.

She was riding high when we first met her (newly engaged, and up for the top police job in the East Anglia area). However, her future and her relationship are left uncertain at the end of episode six.

Read more: Unforgotten season 4 finale delivers twist ending – did Cassie die?

Visit our TV Guide or find all the latest news on our Drama hub.