Your older Apple 12W Power Adapter can be used with your iPad Air4 and your “new” cable - but consider the following…
Your iPad Air4 was supplied with a 20W USB-C Power Adapter and a 1m charging cable. Be aware that your USB-C Power Adapter supports USB PD (Power Delivery) Fast Charging - which dramatically reduces charging time.
Whilst you can use other Power Adapters that feature the older USB Type-A connector, the maximum charging rate will be limited to 12W (i.e., 2.4A @5V).
The simplest way to continue to benefit from Fast Charging is to substitute your 1m USB-C cable - for a longer cable:
https://store.apple.com/uk/xc/product/MLL82ZM/A
Alternatively, if you still intend to use Power Adapter with a USB Type-A connector (accepting the reduced charging rate) you will need a USB Type-A to USB-C cable (you may have already purchased something similar):
- mophie USB-A Cable with USB-C Connector (2 m)
https://store.apple.com/uk/xc/product/HN8A2ZM/A
- mophie USB-A Cable with USB-C Connector (1 m)
https://store.apple.com/uk/xc/product/HN892ZM/A
As you’ll have to purchase a cable anyway, why not just obtain the longer (and otherwise correct) USB-C to USB-C cable?