It was originally published as "The Sign of the Four," but was later collected as a volume (having been serialized) under the title "The Sign of Four." So neither of you are wrong, exactly!
Some type-ins would help. I tried keep bees, apiarist, apiary, etc. Is it necessary to have to type all the articles, such as "a" and "the?" Also, it is not explicitly said that he wears a deerstalker in the books. He wears "a close-fitting cloth cap" and "an ear-flapped traveling cap."