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What Amazon Kindle? Here's an Open Source eBook Reader
(itsfoss.com)
This is a most excellent place for technology news and articles.
There are A4 e-readers. Bamboo note for example.
Yes I've seen one. Maybe there are more now than when I looked, but at that time there weren't enough to be able to make a choice as a consumer.
Onyx Boox has the Note Air 3(10in) and the Tab X(13in). They aren't cheap though, $400 and $900 respectively, mostly because of how expensive those large eInk screens are.
Well an A4 is over 14" diagonally. A 13" might do but 10" doesn't qualify imo. And yeah the one I saw was stupid expensive.