libgtk-4-0 is not packaged on Ubuntu, and on Debian (experimental
channel) it refers to an in-development version 3.91.2.
libgtk-4-1 refers to an real 4.x release which is more useful.

Totally not nudged by Emmanuele Bassi on Matrix.
Also arunkumar413 on Matrix totally did not made me look at package
listings.