Hot on the heels of the watchOS 8.4 update (see “Apple Releases iOS 15.3, iPadOS 15.3, macOS Monterey 12.2, watchOS 8.4, tvOS 15.3, and HomePod Software Version 15.3,” 26 January 2022), Apple has released watchOS 8.4.1 with unspecified bug fixes for the Apple Watch Series 4 and later.
It’s unfortunate that Apple didn’t see fit even to hint at what sort of bugs watchOS 8.4.1 fixes, but given the speed at which Apple released it after watchOS 8.4, you should probably update sooner than later. We would guess that watchOS 8.4 introduced some new problem that Apple felt was sufficiently important to warrant an immediate update, if not important enough to describe.
You can install the 63.8 MB update (for an Apple Watch Series 4) in the Watch app in Settings > General > Software Update. The Apple Watch must be on a charger and have at least a 50% charge to install the update.