One version of the title is The Four Million and Other Stories (Doubleday), however, there is no short story titled The Four Million; rather, it is the title of the second collection of O. Henry's stories, collected in 1905. The book's title refers to the then population of New York City where many of the stories are set. The explanation for the title is given as "Not very long ago, someone invented the assertion that there were only "Four Hundred" people in New York who were really worth noticing. But a wiser man has arisen - the census taker - and his larger estimate of human interest has been preferred in marking out the field of these little stories of the "Four Million." WorldCat confirms that the story list is identical to the original list for that ISBN.
There is a kindle edition titled "The Four Million and Further Stories of the Four Million (A Collection of 45 Short Stories," which also contains the 25 stories in his third collection "The Voice of the City."