Just received the ES4 and... they’re great, straight out of the box!
It has some characteristics of the ZS10 but it’s tuned way better. No overdrawn upper mids, but it has the warm sound the ZS10 has. The bass is lovely fat AND tight. The airyness some KZ’s just nail is once again present in this one. I popped them into my ears and enjoyed them straight away.
Compared to the ZSR, the ZSR’s bass is a little more muddy, and it also sounds a bit smoother. That’s not a con. It’s something you may prefer or not. The ES4 doesn’t have as much sibilance.
Another success for KZ, so soon after the ZSR. Insane!