The NEWS file in the GTK code suggests that it should be possible to build GTK4 without sassc, going all the way back to version 4.0.2. Nevertheless, it seems that the build still depends on it.
Got ii. I managed to build sassc and then build GTK4, but it was flaky. dart-sass (if I understand correctly) would be even worse for me, as, well, it requires Dart. Looks like the tarballs are the way to go.