Local Wiki on readers could be the great place to start:
https://wiki.mobileread.com/wiki, also don't forget that the Universe of ereaders does not limit itself to only four readers that you mentioned)))
There are lots and lots of independent readers that, some say, are better than those "big guys".
Some of them cold be found at
www.ereadersandtablets.com
I would be more than happy to answer your all your questions about independent readers.
Your SharonFruit