According to Wikipedia, the processors are different between the PW3 and PW4.
K2, K3, Voyage, Oasis 1, Kindle 8th gen: Freescale i.MX6 SoloLite 1 GHz
K4: NXP 6SLL 1 GHz
https://en.wikipedia.org/wiki/Amazon_Kindle
The clock speed is the same, but it does not mean performance is identical.
One difference is that the K4 supports Invert Black and White and K3 does not. Another is that K4 has Power Save feature, no other Kindle has this (yet).
Normally they back port features when the hardware supports it, so I infer that these features are built in to the CPU or some other chip that has shown up.
It’s probably my imagination, but I think my PW4 is a teeny bit faster than my Voyage. But I have yet to try to benchmark it. I certainly want it to be true.
BTW, have we settled the issue about whether it has plastic or glass panel or whatever that heated discussion was about from the early reviews?