How to identify the MC: This is kind of a quick-and-dirty way to do it, but after the prologue, who is the first character you see? That’s probably the Main Character. In most movies and stories, the writer wants to get you settled into the MC’s point-of-view, so they start by establishing the Main Character and getting you accustomed to their way of life. (You know, like the hometown of Campbell’s hero.) Theoretically, you could introduce the Main Character at the very end of Act 1 and have the Influence Character first, but that’s less likely, I think.
Basically, if you have four different characters with leading roles, which one do you feel the readers empathize with the most closely? Which one is our emotional “in” for the story? Which one do you feel is “you?” Let me see if The Amulet of Samarkand makes sense as an example. (It’s been a while since I’ve read it, so I could be way off.) The story jumps back and forth between the perspectives of the djinni Bartimaeus and the wizard John Mandrake. In fact, I even think the first perspective shown is Bartimaeus’. But Bartimaeus is, well, a djinni, while John Mandrake is a normal human with normal ambitions and desires. Mandrake is almost certainly the MC, while Bartimaeus is the IC.
Speaking of which, the IC. In contrast to the MC, the IC should be a sort of “alien” perspective. When you’re a little garbage-packing robot on the dessicated remains of Earth, a beautiful, svelte robot on an away mission from a distant space colony will challenge your entire perspective. EVE has a very different background than WALL-E, and their relationship teaches them about the struggle between doing what’s right and following orders, all to the tune of “It Only Takes a Moment.” Disney-Pixar loves this method: a character is going about their daily life when suddenly an intruder with an entirely new way of thinking throws everything out of whack.
Another possibility is that the IC is sort of a dark reflection of the MC. In a TV show I’m really fond of, there was this one episode about one of the main characters trying to join a flight team, and her IC was a daring flier with very similar ambitions, only without a drop of loyalty or worry about the safety of others. The disturbing part of this other character’s actions was not that they were so unusual, but that the MC was almost the IC, just with one key aspect changed.
Lastly, of course, you might be able to figure out the IC because they have a close relationship with the MC–you know, the backbone of the Relationship Throughline. 
So if you’ve got four characters running around, but two of them seem to have the most insightful conversations together, that might be your clue that they’re your Subjective Characters.
