Apple today has stopped signing iOS 11.4 following the release of iOS 11.4.1. This mean, users can no longer install iOS 11.4 on their iPhone or iPad.
Apple keeps on releasing the new iOS version with security updates while stopping the older version to keep their OS safe.
So if you are running either lower or higher than iOS 11.4 versions than you can not go back to iOS 11.4 ever again.
The current iOS 11.4.1 which was released a week ago came with some update like improvements to Find My Airpods & HomePod.