Apple is releasing a security update for the Apple Watch Series 4 today. watchOS 5.3.2 fixes a vulnerability where, "A remote attacker may be able to cause unexpected application termination or arbitrary code execution", and it was released for older Watches on September 26.
Spotted by 9to5Mac, watchOS 5.3.2 will only arrive on your device if you haven't updated your iPhone to iOS 13 yet. If your iPhone is already on iOS 13, then your Watch should already be on watchOS 6, or at least that's the update that will be offered. Obviously, this update is meant to be more geared toward those with older iPhones that can't upgrade to iOS 13, such as the iPhone 5s, iPhone 6, and iPhone 6 Plus.
As usual, you can install today's update via the Watch app on your iPhone. Your Apple Watch needs to be charged to 50% to get it, although if you have updates set to install automatically, it should happen while it's charging overnight anyway.