which cable do i need to connect my 2015 MacBook air to my iPad air 4

You will require a USB Type-A to USB-C data cable.

Suitable cables should be available from good computer hardware retailers. Apple market suitable cables from Mophie:

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

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

• mophie USB-A Cable with USB-C Connector (2 m)

https://store.apple.com/uk/xc/product/HN8A2ZM/A

You will need a USB-A cable.