Refer to

https://wiki.gnome.org/Projects/gtkmm/MSWindows

Tool chain

// msys2 mingw64
pacman -S --needed base-devel mingw-w64-x86_64-toolchain

Install gtkmm-4.0

pacman -S mingw-w64-x86_64-gtkmm4
// or
pacman -S mingw-w64-x86_64-gtk4