Or as an alternative, someone interpreted the plastic back of the Paperwhite 4 as meaning the display had a plastic back. Given that only one site has made that claim and several others have stated that it does not have a plastic substrate, I tend to the alternative.
OTOH, during this search spree, I will admit to having gotten a chuckle out of a comparison which gave the advantage to the Paperwhite 4 compared to the Clara HD due to the Paperwhite's "seamless Overdrive integration".