www-apps/drupal-8.0.0_rc4 was released yesterday: https://www.drupal.org/node/2614756
The version can probably be bumped by just renaming the ebuild. However, RC4 does not fix bug #565848, thus I don't have a reference that works.
10:21 < gentoovcs> jmbsvicetto → repo/gentoo (www-apps/drupal/files/, www-apps/drupal/) [www-apps/drupal] Bump to 8.0.0_rc4 release - fixes bug 565814. Drop old beta / rc versions. Update postinstall instructions. 10:21 < willikins> gentoovcs: https://bugs.gentoo.org/565814 "www-apps/drupal-8.0.0_rc4 version bump"; Gentoo Linux, Applications; UNCO; devurandom:web-apps Added to the tree.