How to connect iPad Air 4th Generation to a USB port.

I presume you are attempting to connect your iPad Air4 to a computer that has an older style USB Type-A port. If so, you simply require a USB-C to USB3 Type-A cable. While these cables should be readily available from good computer stores, here is an MFI Certified cable (from Mophie) that is marketed by Apple:

• mophie USB-A Cable with USB-C Connector (1 m)

https://store.apple.com/uk/xc/product/HN892ZM/A