Viewers have previously been left reeling from the tragic deaths of popular characters Lady Sybil and Matthew Crawley, with speculation rife as to what the future holds for their bereaved loved ones.
The new season picks up six months after Matthew's death in a car crash and the exit of the actor who played him, Dan Stevens.
Lady Mary is left to bring up baby George alone, following her husband's fatal accident while on his way to the hospital to see his wife and their newborn son.
Also returning for the fourth series will be chauffeur-turned-journalist Tom Branson, played by Allen Leech, who is also bringing up a child alone - little Sybbie - after the death of her mother Lady Sybil.
What a roller coaster you have in store, but I'm sure you'd expect no less!
Allen Leech, Downton's Tom Branson
Meanwhile, newcomers include Tom Cullen as old Crawley family friend, Lord Gillingham.
The dashing gentleman visits Downton for a big house party, along with his valet Green (played by Nigel Harman) and tries to give Lady Mary advice on inheritance and the Downton estate.
Then there's Charles Blake, played by Julian Ovenden, who is set to helping big estates like Downton to run more efficiently.
He appears alongside Evelyn Napier, played by Brendan Patricks - who will be remembered by fans for bringing the ill-fated Turkish diplomat Kemal Pamuk to Downton - and instantly gets on the wrong side of Lady Mary.
As has previously been reported, Downton is also set to see its first black character introduced - in the form of charismatic jazz singer Jack Ross, played by Gary Carr.
From Chicago, he sings at the Lotus jazz club in London which is visited one night by Lady Mary, the vivacious Lady Rose, Branson and Lady Rosamund.
Other new cast members will include Dame Kiri Te Kanawa, Joanna David and Dame Harriet Walter.
Downton has also been longlisted by the Radio Times as a contender for the title of greatest British TV drama of all time.
But it's up against tough competition from the likes of broody ITV crime drama Broadchurch, the BBC's Sherlock and Doctor Who and classics like the original House of Cards.
A total of 20 programmes have made the list, before a winner will be chosen by the public.