Has anyone noticed rapid battery decline since installing iOS 16.4.1 ?

Pretty much every iOS upgrade results in battery drain comments. Go through your Settings and see if it resulted in a feature being activated by default and then decide if you really need that feature turned on.

A small minority of users report “rapid battery decline” after EVERY iOS update. Simply restarting the device often clears the problem. There may also be basic routines running after an update in the background that slurp up battery for a while. In either event the issue almost always resolves in a day or two.