Here I found some mention about those packages being released in next release of Ubuntu - is it so?
Whatever is going on, it would be nice to write it on the GTK4 installation page next to the table with those packages, since this is really confusing for beginners like me.
I tried compilation from source, but there are two places to download GTK4 (download.gnome and ftp.acc.umu.se) - the first I did not manage to compile, the second compiled, but there were many errors at the meson test stage and many “subapps” in the gtk-demo app did not work or crashed the app.
Again - whatever is going on, clear instructions on how to install/compile GTK4 are important…
The working of Debian packaging is completely different. All the released packages get into Debian Sid (Unstable). After being tested it gets into Testing (approved for stable but still may have bugs), then after some noticeable months, it gets to Debian Stable.
From Unstable to Testing happens almost within three weeks. And Linux Mint is based on Ubuntu which is based on Debian Testing. So for LM to get the packages, you need to wait for Ubuntu to make a new release that contains GTK 4 and then wait for Linux Mint team to pull from Ubuntu.
So you can use Flatpak or a faster updating system like Debian Testing, Fedora or Arch or compile from source.
I prefer Flatpaks here.
Thank you all. Most of your replies are answering my problem.
At the end, I installed Gnome Builder from Flatpak, which “should” include the 4.0 version of libgtk. For now, tutorial examples compile with libgtk4.0 flags.
(Although I’m not sure whether is used the library that came with Gnome Builder or the library that I compiled and installed from source with meson and ninja (and now I’ve got no idea how to uninstall it…))
I’ll probably post some more questions about this topic.