[gnome-kit] dev-cpp/gtkmm-4.21.2 doesn't emerge

```
>>> Configuring source in /var/tmp/portage/dev-cpp/gtkmm-4.21.2/work/gtkmm-4.21.2 ...
meson --buildtype plain --libdir lib64 --localstatedir /var/lib --prefix /usr --sysconfdir /etc --wrap-mode nodownload -Dbuild-demos=false -Dbuild-tests=false -Dbuild-documentation=false /var/tmp/portage/dev-cpp/gtkmm-4.21.2/work/gtkmm-4.21.2 /var/tmp/portage/dev-cpp/gtkmm-4.21.2/work/gtkmm-4.21.2-build
The Meson build system
Version: 1.8.5
Source dir: /var/tmp/portage/dev-cpp/gtkmm-4.21.2/work/gtkmm-4.21.2
Build dir: /var/tmp/portage/dev-cpp/gtkmm-4.21.2/work/gtkmm-4.21.2-build
Build type: native build
Project name: gtkmm
Project version: 4.21.2
C compiler for the host machine: x86_64-pc-linux-gnu-gcc (gcc 12.3.0 "x86_64-pc-linux-gnu-gcc (MacaroniOS 12.3.0) 12.3.0")
C linker for the host machine: x86_64-pc-linux-gnu-gcc ld.bfd 2.40
C++ compiler for the host machine: x86_64-pc-linux-gnu-g++ (gcc 12.3.0 "x86_64-pc-linux-gnu-g++ (MacaroniOS 12.3.0) 12.3.0")
C++ linker for the host machine: x86_64-pc-linux-gnu-g++ ld.bfd 2.40
Host machine cpu family: x86_64
Host machine cpu: x86_64
Program python3 found: YES (/usr/bin/python3.9)
Found pkg-config: YES (/usr/bin/x86_64-pc-linux-gnu-pkg-config) 2.2.0
Dependency gtk4 found: NO. Found 4.21.2 but need: '>= 4.21.4'
Found CMake: /usr/bin/cmake (3.31.10)
Run-time dependency gtk4 found: NO (tried pkgconfig and cmake)
Looking for a fallback subproject for the dependency gtk4
ERROR: Subproject gtk4 is buildable: NO

meson.build:134:10: ERROR: Automatic wrap-based subproject downloading is disabled
```

GTK4 4.21.4 is required.

Start with the gtkmm-4.21.2 build log and the failure at meson.build:134:10. Check how the package declares its GTK4 dependency against the Meson requirement, then verify that GTK4 4.21.4 is selected and the gtkmm package emerges successfully.

Written by the indexing model from the issue text.