Oh it comes with custom firmware? This is very interesting. I would love to be able to modify some UX and I am sorry, but I need to get the following out of my head. All the e-readers I have had have made it impossible to turn off features like:
1. Selection highlighting... I never use highlighting when reading fiction, but whenever I am not careful enough when turning a page, it'll go crazy with highlighting. Flashing screen, need to close the popup that has added the highlight, removing the highlight again etc.
2. Most of the time I don't want to click on a word to find out its meaning. It's sometimes useful, but I'd rather have it under menu to temporarily enable it. Same reason as before. My e-readers tend to prefer this often enough rather than taking the "next page" action.
3. Make "previous page" be small and not-under-my-finger. Ideally let me choose its position in a fairly precise way.
4. Easy access to accidental "scroll to page 900". I generally don't want it to happen and to be honest, I struggle to think of anybody who does. It can live in a single tiny faraway menu that is impossible to accidentally tap.
5. Swipe-left for previous page. It almost never happens when I want it to happen, so I'd rather turn it off.
Personal request to any e-book reader software engineers. Please save the position in the book to persistent storage on each page change or every few. At least if the e-reader has any chance of crashing at all, which has been the case with all the ones I have ever had. Yes, not all of them save it...
That's not to say that all the above things are universally bad UX. I think many of these are very useful, if reading non-fiction or having a different goal when reading such as learning a new language. It's just that they are less than brilliant if the goal is to read a book for entertainment in the most comfortable way possible with the fewest things going wrong by accidental taps.
It doesn't not come with custom firmware and xteink tried to block it citing hardware issues. Key word is tried and it's still very easy to install custom firmware.
The X4 doesn't have a touch screen, so you're safe on those points. The next iteration will do though, although I think you will be able to turn it off in the reader view.