It would be awesome add a category for 4K/UltaraHD wallpapers in your site....
You can also create a section in your site for most popular wallpaper of the month or week as like http://wall.alphacoders.com
Regarding the "UltraHD" category: we already have it!
A lot of members asked me to add a "top popular wallpapers of the month/week" section, I think it's a good feature and I'll probably add it in the future, but don't expect it in the very near future.
I cannot find the category for 4k/ultrahd.
Click here. Additionally you can search wallpapers using any custom resolution possible.