Upgrading minor versions of Qt seems to be a hassle, as each version blocks the previous one, and because of interdependencies, Gentoo can’t understand by itself how to solve them. The solution is to tell it to specifically upgrade all installed modules.
sudo emerge -avu1 $(qlist -IvC /qt- | grep 4. | sed s/-4.*//)
This will allow Gentoo to solve the dependencies by uninstalling the old versions, unlike when you only try to upgrade a single package like x11-libs/qt-core.
Life saver! Even 3 years later š