The Emmy-winning period drama will return to ITV1 in autumn 2013 with eight new episodes.
A special extended episode will also air at Christmas 2013, with filming set to begin in February next year.
Gareth Neame - exec producer - said: "Viewers can look forward to more drama, comedy, love, hatred, jealousy, rivalry, ambition, despair and romance."
Laura Mackie, Director of Drama at ITV, added: "We're thrilled to welcome back a drama series that has become a much anticipated part of all our lives every autumn and achieved success around the globe.
"Creator, writer and executive producer Julian Fellowes and the production team, led by Gareth and the producer Liz Trubridge, never rest on their laurels and have exciting plans for the fourth series."
Downton Abbey will also return to ITV1 this Christmas for a two-hour festive special, to air on December 25 at 9pm.
"We hope the Downton family of viewers will gather together to watch this major TV event as a shared Christmas night experience," said Mackie.
"Last year's special episode set the nation talking on Boxing Day and 2012's Christmas Day episode is no different.
"As ever, Downton will take its audience through a whole range of emotions. All of life's experiences will feature in this episode. This is one episode you will want to see live with millions of others - and best have the tissues ready."
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