iOS 14.4. Quick Edit wont open

Strictly speaking, it is possible to restore the immediately preceding [point] version of iOS/iPadOS - but only whilst the previous version is remains digitally “signed” by Apple. 

After a “new” version is released, the immediately preceding release continues to be available for a few days - this providing a short overlap in time in which to identify a major/critical issue prior to revoking the signing-certificate of the prior release. As such, it is technically possible to “revert” to the immediately preceding release during the early days of a an update. 

After the update has been demonstrated to be free of major issues - that had not been identified during beta-test - the signing-certificate of the previous release is revoked by Apple. It is at this point the ability to restore the previous release is lost. You should note that the window of opportunity is not linked to the point-in-time that you install and update - but instead is a function of how long Apple are prepared to leave the opportunity open after a new release. This window of opportunity is typically days, not weeks.

At this time, iPadOS 14.6 is currently the only signed (available) version of iPadOS. Apple revoked iPadOS 14.5.1 certificate signing on 1st June.

Have you tried restarting the iPad Air?

There is no official way to downgrade your iPadOS version.