It’s safe to assume that Glade will neither be ported to nor support GTK 4. Juan Pablo (Glade maintainer) posted a video of his prototype for a new tool
https://blogs.gnome.org/xjuan/2021/02/28/cambalache/
and something on twitter
@chergert added ui design marked with “High Priority” to the goals for GNOME Builder 41
Right now it looks like both projects will happen independently.