Amazon.com has added season four of British period drama “Downton Abbey” — the most-watched TV series on its Prime Instant Video service — with all episodes available today to members of the free-shipping program.

Prime Instant Video is the only video-subscription service that offers all four seasons of “Downton Abbey.” In addition, under Amazon’s pact with PBS, the SVOD service will continue to be the exclusive subscription home through season five of the series, a Carnival/Masterpiece co-production.

SEE ALSO: ‘Downton Abbey’ Renewed for Fifth Season

“Downton Abbey,” the popular pre-World War I British drama, has won multiple Golden Globe, Primetime Emmy and Screen Actors Guild awards.