I know this is a four years old topic, still It had the solution for a problem of today.
I tried to sideload a set of books originally in epub format using the reliable method of silent converting by Calibre. The books however were unable to open on the device, even if the ebook reader in Calibre did handle it well.
The resoulution of the problem was (as suggested in this old topic) to
use KF8 (AZW3) instead of mobi.
So thanks @JSWolf, this comment helped me after an hour long trying and confusion.
(I still don't understand why but at least it works, however I will most probably never fill up the 32GB memory of my PW4 with text e-books even if I were to use double format files)
Some answers of possible future questions:
Yes, I had used mobi as default format for PW4.
Yes, I have changed it now to azw3.
