iPad Air 4

These support pages should help with troubleshooting battery and charging issues:

Charge and monitor the iPad battery - Apple Support

If your iPad won't charge – Apple Support

Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. 5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow.

Be aware that many computer USB ports cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).

If you continue to experience difficulties when using an appropriate Power Adapter of 12W or greater, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, 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/gb/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 test and assess your iPad, Power Adapter and cable(s):

Genius Bar Reservation and Apple Support Options - Apple

For iPad models with a Home button, do a simple hard reset of your iPad by holding down both the Home and sleep/wake/power button simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

OR

For much newer design iPad models, without a Home button, try the new simple hard reset procedure for the new 2018/2020/2021 iPad Pros, 2020 iPad Air 4 and 2021 iPad Mini 6 models.

1. Press and release the volume up button.

2. Then immediately press and release the volume down button.

3. Then, press and hold the single sleep/wake/power button You will see the Slide to Power off button, but keep on holding down the power button until you see the Apple logo, then let it go.

Once you have performed all the steps, wait for a few seconds and your 2018 iPad Pro will boot up completely to the iPad Pro’s Lock screen.

If no success,

Try other known good, compatible power/charging cables and/or other known good 18 or 20W power/charging adapter blocks to test for current cable and/or power/charging block failures.

Power/charge cables and/or power/charge adapter blocks can AND DO fail at any time for any reason.

If no spare power/charging cables or power/charging adapter blocks or NO joy on getting this iPad charging, you can purchase new cables and power/charging blocks from your nearest, “local” Apple Store location or from Apple's Website

OR

Purchase third party Apple MFi ( Made for iOS devices ) certified power/charging cable and adapter block, from online retail, mobile device sellers, to test with, in order to determine that your existing power/charging cables and adapter block is bad/faulty.

OR, if no success with brand new power/charging cables and/or power/charging adapter blocks,

You may need to use the “Get Support” link, at the top far upper right hand corner of every Apple Support Community Webpage, and make an appointment for your nearest, local Apple Store and bring your iPad and charging cable and adapter block, to the appointment, for Apple employees to test/diagnose and options presented to you.

Best of Luck to You!