iOS 17.4 update, image mirroring function lost

Addressing your last question first - you cannot roll-back to an earlier version of iOS/iPadOS.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed. iPadOS 17.3.1 and earlier are no longer digitally signed - and cannot now be installed.

Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data. 

As for your other issue concerning screen mirroring, the nature of your issue isn't entirely clear. Perhaps you can provide more specific description of precisely what you are attempting to achieve - and what happens when you try.

With the benefit of better understanding of your problem, appropriate guidance may be possible.

Begin with a forced-restart of your iPad - as this will often clear an isolated software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If the problem persists after performing the restart, it may be necessary to reset your iPad settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPad settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPad to factory settings - after which you'll need to restore your data from your backup:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Otherwise, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to assist:

Genius Bar Reservation and Apple Support Options - Apple

I stream media content over AirPlay from an iPad Pro with iPadOS 17.4 to a TV without issue.

Are you having difficulty with a particular streaming service - or are you unable to play any media at all via AirPlay?

Are you attempting to connect to your TV wirelessly using AirPlay - or via a wired connection via an appropriate adapter (e.g., HDMI) between your iPad and TV? Something else?

I'm delighted to learn that you've worked through and resolved the problem.

I have been unable to mirror my portable monitor (Arzopa) to my iPhone or iPad with iOS 17.4.1. This is very annoying apple!