I bought a 6 inch model for $60 (it's now double the price) from another brand a couple months ago and I love it. A 4 inch screen is just too small IMHO.
I considered buying an X4 for the portability (and to be honest installing and tinkering with crosspoint sounds fun).
But the kobo can fit in my jeans pocket and is about the size of a normal paperback page. I feel like the X4 would be too small and might be frustrating.
I’m on Calibre Web and Shelfmark.