Just received iPad Air 4th generation. I have 3 older iPad with 3 12W USB power adapters and numerous cords. Is there a converter which will allow me to use these cords and adapters with my new iPad Air 4th generation product?

Yes it’s a USB-A -USB-C adaptor, came across it in the Apple online shop when researching before I bought my iPad Air 4. And fake cart shopping of all Apple products. USB-A is the original USB, USB-C is the new school. You could probably buy it anywhere that sell tech, 100% at Apple store and Apple online in the wire/cables section, to get to it go into any product and scroll all the way down then more bolder icons of Apple products genres will appear.

You can use the following cable with older USB power adapters: https://www.apple.com/shop/product/HN892ZM/A/mophie-usb-a-cable-with-usb-c-connector-1-m

or use the following adapter: USB-C to USB Adapter - Apple

Also note that the iPad Air 4th generation should have come with a 20W power adapter and USB-C to USB-C cable.

I also just received the iPad 4th generation and my adapter is 20w. It charges full in about 2 hours

You get a 20 watt charger and a cable in the box.