The fourth incarnation of Doctor Who was played by Tom Baker, broadcast from 1974 to 1981.
The Fourth Doctor's costume was inspired by a painting of French artist Toulouse-Lautrec. The now legendary scarf was a happy accident - the result of a freelance knitter not realising she didn't have to use all the wool she had been given.
There are several patterns available for the scarf, even a history of its various incarnations. The best site is Doctor Who Scarf, and they have the pattern for the only scarf version I recall, the season 18 scarf, complete with photos. They have all four scarf patterns, a nicely put-together site.