The link is hidden in the Store (being a beta) but I have it for you here: https://www.microsoft.com/en-gb/p/mytube-beta/9wzdncrdt29j - so go knock yourself out.
As you'd expect, I've given v4.0 a brief whirl and done some screenshots:
The hamburger menu looks a bit oversized on Mobile, but this is an early beta, so it'll get fixed! Note the hierarchical, collapsible options; (right) browsing videos in my subscriptions in the usual way.
Searching for a text string returns the usual thumbnailed matches, all very slick; (right) playback in action, with the usual tabbed split between details, comments, etc.
Handy to have my subbed channels in the hamburger, expanded pane; (right) videos carry on playing in the background and with full integration into Windows media playback - super handy for music videos, as here!
As I say, I'll come back to this in much more detail when it exits beta, in a few months.
PS. The regular v3.x, non-beta myTube UWP is also available, and in fact recently updated, details here.