X4 is great - has usb-c charger and with the cover feels like an easy to pocket and bring everywhere reader. Does not fit on the back of an iphone. I assume the magnet layout works with popular android phones though.
X3 is also great, actually fits on the back of the phone with the magsafe - magnet is a _bit_ too weak. It does fall off in my pocket frequently but I haven't lost it yet. Does not have usb-c - has weird little magnetic 4 pin charger.
I will gift my X4 to my brother once I've loaded some more content to it.
I recommend this for anyone that wants to save web novels for offline reading https://github.com/lncrawl/lightnovel-crawler - Calibre will sync the epubs to the x4 and x3.
Use the crosspoint OS https://github.com/crosspoint-reader/crosspoint-reader
All that said... I do still use my kindle as default. More content accessible more easily, better syncing, backlight