GObject Introspection, how to suppress GTK4

[Stefan Salewski <mail ssalewski de>]

I have a Nim gintro user who has both GTK4 and GTK3 installed on his
box, and seems to have some trouble: 

https://github.com/StefanSalewski/gintro/issues/30

I can not test that, because for my Gentoo Linux box only GTK3 is
official available.

My gintro package should generate latest GTK3 bindings for Nim
programming language, and ignore GTK4 for now, as I am not able to test
GTK4, and without testing there is no chance for working GTK4 bindings.

But it looks like he is getting GTK4 -- I assume he has a Linux box.

From

https://developer.gnome.org/gi/1.56/GIRepository.html#g-irepository-get-default

I do not see how one can really select GTK version. For most people my
generator program seems to work, because they have GTK3 installed.

I am using g_irepository_get_default() and g_irepository_require() --
the later function gets NULL for version. May an undocumented pattern
work for version? Like a guessed "<4.0" ? Or may a plain "3" give us
latest available GTK3?