Eli Hayes’s review published on Letterboxd:
This review may contain spoilers.
"they are one person"
the individual, e.g. Lena or Kane, and their shimmer-self, are each their own "person" or identity, with their own makeup, refracting off one another and everything else inside & outside of the Shimmer. Lena is one identity, and her shimmer-self is one identity (the Shimmer, as its own, unique entity, being an infinite number of identities), but the more time spent inside the Shimmer, the more these two identities amalgamate and become One with the Other.
"they are two alone"
this is branching off the first point, in the sense that the more time an individual spends within the Shimmer, the more they become Another version of themselves, in addition to the person that they "are," or once were; every individual that enters the Shimmer, over time, whether alone or not, really becomes a being with two overlapping identities.
"they are three together"
once an individual comes in contact with their shimmer-self, there are, in a sense, now three identities, "together," contained in a singular human vessel: 1) the individual that entered the Shimmer, 2) the secondary identity created by the refractions, and 3) the shimmer-self.
"they are for(/four) each other"
the refractions also alter the shimmer-self so, regardless of whether the shimmer-self exits the Shimmer, or the "original" Kane (or Lena) does, the individual exiting will be leaving with an amalgamation of identities.
the "original" Kane commits suicide with a phosphorus bomb, and his shimmer-self then exits the Shimmer through the vessel of Kane's body, but not before also experiencing the effect of the refractions, and bringing part of the "original" Kane back home with it. the "original" Lena, on the other hand, is the one to exit the Shimmer, as the shimmer-self disintegrates (while simultaneously, through the touching/burning of Kane's "original" body, showing the portion of humanity/Lena that it has consumed).
now Lena has returned home but, even though she's not her shimmer-self, she's still a novel version of herself that has been transfigured by the refraction process & the amount of time spent in the Shimmer. they are four: Kane's shimmer-self, the part of his "original" identity that remains, and Lena's modified-self, as well as the part of the Shimmer that remains.