In case anyone is interested, here's a link to the company who makes the screens for this BK4304 (and several other "BK" eReaders). They are a much lesser known competitor to E-Ink, Inc. and they call their technology
O-paper.
http://oedtech.com/English/support/i...?menuid=030301 (Intro)
http://oedtech.com/English/support/i...?menuid=030302 (more info)
http://oedtech.com/English/product/c...enuid=03020101 (products)
In the last link you'll see the BK4304 on the right edge of the picture. You'll also see the O-paper specs. I've read somewhere that they are basically the same as the E-Ink Pearl screens. (Same contrast ratio 12:1.)
Good information page on E-Ink (and O-paper) screens here. E-Carta has a 15:1 contrast ratio.
https://e-ink-reader.ru/display_en.php