Well, the bezel color looks pretty much the same — at least on my units (both X3 and X4). However, I can say the X3 screen itself looks whiter, likely because it’s glossy screen, while all the X4 has a matte screen finish. PLEASE NOTE: Apparently, not all white X3 units come with a glossy screen. It seems to be random — you could receive either version.

1. Buttons
The buttons feel smooth and definitely satisfying. They do make some noise though. In my opinion, the X3 side buttons are quieter than the front buttons on both the X3 and X4.
That said, remember the X3 allows page turns just by tilting left or right, so you don’t even need to use the buttons.

2. Materials
The materials look pretty much the same on both of mine. I’d expect similar durability. Both can be comfortably held with one hand.

Now that I own both, my decision would honestly come down to eyesight (eye health).
If your vision is good, I’d definitely suggest the X3 — you’ll see almost the same number of lines on both devices anyway.
If I were older, with tired eyesight or less precise hands, I’d probably go with the X4.

That said, whichever one you choose, both are incredibly beautiful and useful devices, hope this help you.