You probably read my words about carrying a 5" XD-45 in a Blackhawk CQC SERPA. I love mine!
It is the same holster for both Service and Tactical models. As you noted, the Blackhawk CQC is an open-bottom holster, so it will accomodate either length. And yes, the muzzle of a Tacticalwill stick out the bottom approx 3/4". At the time that I bought mine, anyways, there were not specific models for 4" & 5" guns. And I don't believe that they intend to make a 5" specific holster. If you want a similar holster with a closed bottom, check out Bianchi Carry-Lok.