How to do iPad mini 4 Screen Mirroring with MacBook Air (13-inch, 2017) running ?

it's unclear if you wish to use the ipad as a second display for the mac if so read

Use your iPad as a second display for your Mac with Sidecar - Apple Support

or if you wish to have the mac show what the ipad is showing in which case read

How to Mirror Your iPhone Screen to a Mac Computer (businessinsider.com)

then click the first blue text

be mindful of the requirements

Sidecar system requirements

Sidecar requires a compatible Mac using macOS Catalina or later and a compatible iPad using iPadOS 13 or later.

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2017 or later, or iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019
•   
• iPad Pro: all models
• iPad (6th generation) or newer
• iPad mini (5th generation) or newer
• iPad Air (3rd generation) or newer

Additional requirements

• Both devices must be signed in to iCloud with the same Apple ID using two-factor authentication.
• To use Sidecar wirelessly, both devices must be within 10 meters (30 feet) of each other and have Bluetooth, Wi-Fi, and Handoff turned on. Also make sure that the iPad is not sharing its cellular connection and the Mac is not sharing its Internet connection.
• To use Sidecar over USB, make sure that your iPad is set to trust your Mac.