Downton Abbey could go on until 2020, says executive producer Gareth Neame, 46, told AFP that the period drama series, whose fourth series premiered in the US at the weekend, would end eventually, but he didn't know exactly when. READ: A review of the Downton Abbey 2013 Christmas special But the executive producer said that ending Downton would be a creative decision. Clearly the show is an economic success. But to me, the more important thing is to make sure it is well-formed, comes to an end at the right time, and is not lured into the usual thing of trying to keep something going past its time. Neame, who heads the British production studio behind popular series such as Poirot and Whitechapel, also said he had been surprised by the worldwide success of Downton Abbey - particularly in the US and Spain. I was hopeful that we would have the usual 50-plus, upscale, anglophile American audience, but I didn't know that we would become such a mainstream hit. "Why we should be the most popular non-Spanish program in Spain, I don't know," he added. The fourth season of Downton Abbey, starring Hugh Bonneville and Michelle Dockery, finished in the UK in November, while a Christmas special aired on Christmas Day 2013. GALLERY: Downton Abbey's most shocking moments