I struggle with this issue of the indefinable "inequity" of a story as well, because it always risks reducing the concept of inequity to being irrelevant within the actual practice of writing the story – If I can't as an author explain what it is, then I'm unlikely to do much with it when putting text on the page. However I think your mention of SUBJECT might be particularly helpful here in distinguishing the inequity as defined by four different points of view versus what those four points of view are struggling with.
In Four Weddings and a Funeral, it sure seems like everyone is wrestling with marriage, specifically, finding someone to be attached to before you die. There are lots of different conflicts around marriage, and varying points of view, but all those points of view are wrestling with the question of marriage and whether to settle or wait for the right person. Whiplash seems to be about excellence, what price you should pay to achieve it, and how to go about paying that price. It's not about four different perspectives on becoming a great musician. It's about what it means to become a great musician, and that subject is illuminated by looking at it from four different perspectives.
I imagine that in some stories the subject matter is somewhat more abstract, and thus you can have an MC dealing with a situation of being (in my favourite failed example) stuck in a well, while everyone else is dealing with who's going to become mayor. But somewhere in there is a subject matter that all four perspectives are dealing with.
Assuming this is correct (and I'm sure I'll get my well-deserved beating from @jhull if it's not), then for me as a novelist this kind of solves the cognitive dissonance of the indefinable inequity (or rather, the inequity only defined by a theoretical 'fence' set by four perspectives). When I sit down to write a book, I ask myself, "what do I want to write about" and the various perspectives emerge from that. I never sit down and think, "what four abstracted perspectives do I want to write about" and then seek out a subject. Dramatica's value for me always comes in when I think, "I want to write about marriage. Or swordfighting. Or gastric bypass surgery" and then, "Okay, well I'd better have four different perspectives on that subject in order to explore it meaningfully."