The other four are 5 Johnny Winter 4 John Mayall and the Blues Breakers ft E. Clapton 2 Junior Wells and Buddy Guy - Hoodoo Man Blues 2 Paul Butterfield Blues Band 1 Moanin' in the Moonlight - H. Wolf
Moanin' in the Moonlight is Howlin' Wolf's first 12" LP. The John Mayall LP with Clapton is their only collaboration. Mayall had an LP prior to that, but "Bluesbreakers"does not appear in the title of that LP. Hoodoo Man Blues is the debut LP featuring JW and BG as group leaders.
Actually my understanding is that "Howlin' Wolf" was first on shelves? And of course Sam Phillips had released 45's on him and promoted him in a big way for a couple of years. "Bluesbreakers" as a group is less a band than Butterfield's was; much more of a revolving door. Mayall debuted on "Mayall plays Mayall" or whatever it was called, and Clapton had been a founding member of the Yardbirds and sold a few million copies that way. Wells had been on a couple of Newport collections and Guy had made a whole raft of sides earlier; although those weren't collected on a double cd until much later, still they were available for sale in record stores... And Winter was already known to aficionados through the "Blues Experiment" album, which came before. So what counts as a debut?
Winter's debut, the eponymous release with "Be Careful With a Fool", etc, is his debut LP. Moanin' in the Moonlight is the Wolfman's first vinyl 12" release. JW and BG may have played together and separately b4 releasing Hoodoo Man Blues, but it's their first LP together. I'm sure you know that Mayall had a group with Huey Flint, John McVie and others prior to the LP with Clapton, prior to the bassist and drummer forming Fleetwood Mac, and Peter Green replaced Clapton for a number of concerts then became a member of Mayall's group and appeared on A Hard Road. As far as what constitutes a debut album, I would define it as the first release by a recording artist as the leader of his group or the first release by a group, like The Beatles or Earth, Wind and Fire, which may or may not have a leader, per se. From that perspective, the Mayall LP isn't a debut, but the others are.
https://www.google.com/search?q=howlin' wolf discog&ie=utf-8&oe=utf-8#safe=strict&q=howlin' wolf discography http://www.furious.com/perfect/wolf/discography2.html