But it is not the "4th film" by QT!

The tagline refers to the films written and directed from start to finish by him. If we have to play the smart asses, then we should add the 1/4 part of “Four Rooms” and the half unfinished “My best friend’s birthday”. So i guess 4 + 1/4 + 1/2 = “The 4th and 3/4 film by Quentin Tarantino”.