The reason for the double release is not the miscompile but further upstream kernel fixes to the SACK vulnerability patches after 18.06.3 got tagged.
Apparently the first iteration of SACK fixes broke at least steam-client on Linux, prompting another Kernel release with just one commit: https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.131
18.06.4 was done in response to that Kernel bump since 18.06.3 was already out in the wild and we didn't want to alter/reissue the Git tag.
Release notes, announcements etc. will follow within the next days.
18.06.3 and 18.06.4 are two ordinary service releases that just happened to happen very close to each other (before I had the time to do release notes).