> ...
> So: what is the current best way to do gtk3 (4?) from guile?

You should (highly) consider using Adwaita instead (libadwaita),
and gtk-4 for what ever isn't customized (yet) in libadwaita.

But to address your question (disclaimer, I am its maintainer), as Mike
already did suggest, you should use g-golf [1].

Regards,
David

[1]	which by the way comes with gtk-4 examples as well as a port
(wip) of the upstream adwaita-1-demo. Images (screenshots) of those
examples here: https://www.gnu.org/software/g-golf/learn.html