Apple has stopped signing iOS 12.1.3 which means downgrading back to it from iOS 12.1.4 is no longer possible.
iOS 12.1.3 was the last major version of iOS that was still being signed by Apple which had the infamous Group FaceTime bug. Given that iOS 12.1.3 cannot be jailbroken, I doubt there’s any reason for someone to downgrade back to it from iOS 12.1.4, especially since the latter fixes the Group FaceTime bug.
For users looking to jailbreak their iPhone or iPad, they need to downgrade it back to iOS 12 – iOS 12.1.2. However, Apple is no longer signing them so there’s no way to downgrade right now if you don’t already have SHSH blobs saved.
Apple had released iOS 12.1.4 in the first week of February so the company kept signing iOS 12.1.3 for an unusually long time.
Right now, with iOS 12.2 still not released, iOS 12.1.4 is the only release that is being signed by Apple. If you are looking to downgrade your iPhone or iPad to iOS 12.1.3 for some reason, it would only be possible if you had saved the required SHSH blobs. Otherwise, you are out of luck here.