Can I use a MacBook Pro charger for iPad Air 4?

Yes.

No issues, but your iPad Air will NOT charge any faster than it does with the Apple 20W charger.

The rate your iPad charges at is designed in and will not ever charge any faster.

Best of Luck to You!

You can safely use a Macbook USB Power Adapter to charge your iPad.

You should note, however, that the iPad will not necessarily charge any faster. The iPad will only charge at its maximum supported charging rate of 18W - when using a USB PD (Power Delivery) Power Adapter.