Are you sure? Because all four hairs share the same exact array of textures from what I’m seeing (except for Nightlife, seeing as how it turns out I actually don’t have it yet). But you can really see the design similarities between all of them, especially with Catwalk and Charm.
Just because they have the same color array doesn’t mean they have the same textures. After all, most of the more recent DAZ Original hairs offered through Platinum Club (Philly, Elements, Tears, Love Me, Swim Cap, Minnie Bow, a few others I can’t think of) have the same color array, and don’t seem to be particularly interchangeable.
And yes, I do have all of those, and after I read what you wrote, I tried the textures back and forth. PH Charming’s and PH Catwalk’s textures leave big blank strips on Charm where the textures don’t have the same material zones, and vice versa. Nightlife and Fashion are sorta kinda interchangeable, but turn into different hairs when you do that. (Not necessarily a bad thing, although I think using Fashion textures on Nightlife doesn’t work at all—I’m not at that computer and can’t test it right now.) I didn’t test every single one of them, although I did do a few more, and most of them just didn’t work at all. There’s a fair amount of similarity, yes, but they really are different.