The ES4 has a 10mm graphene DD while the ZSA has an 8mm DD which may or may not be graphene.
An 8mm DD may indicate that the ZSA could have a very balanced signature if it is tuned like the ZST's 8mm DD though the ES4 is the one advertised as having a "reference" sound signature. In order for the ES4 to have a "reference" sound signature the crossover must have been put to good use because the 10mm DD is quite full-bodied in it's natural state (ZSR & ES3) but, apparently, KZ wants to get the point across that the ES4 is not an entry-level in-ear monitor; it is professional grade.
The ES4 appears to have a customized 30095 treble BA while the ZSA has an "improved" BA (not necessarily the same BA advertised on the ES4).
This will be the first time hearing the "improved" BA so it's useless speculating at this point. That said, the ZSA's triple-vented faceplates may afford them a bit of air and an improved sense of space between instruments.
My expectation is that the ES4 with it's 10mm graphene DD and a proper crossover is a direct upgrade to the already good ES3.
....and as always, further refinement (of the ES3) is always welcome.
Haven't heard either so remember.......It's all conjecture.
Also, I think it's great that the cable offered with the ES4 is different from the cable offered with the ZSA. I have a handful of identical KZ cables from the ZST and ES3 so more variety is a good thing.
Have to wait and see but I'm all ears.