MacRumors published a story that Apple has released the fourth betas of iOS and iPadOS 15.1.
Apple Seeds Fourth Betas of iOS and iPadOS 15.1 to Developers
Apple today seeded the fourth betas of upcoming iOS 15.1 and iPadOS 15.1 betas to developers for testing purposes, one week after seeding the third betas and three weeks after releasing iOS 15 and iPadOS 15 to the public.
Apple Seeds Fourth Betas of iOS and iPadOS 15.1 to Developers
iOS and iPadOS 15.1 can be downloaded through the Apple Developer Center or over the air after the proper profile has been installed on an iPhone or iPad.