I have a IPad Air fourth generation can I get OS 13 on my I pad.

The iPad Air 4th generation was released with iPadOS 14 and cannot install any iPadOS version before that.

The original iPad Air cannot be updated beyond 12.5.1

The 2nd and 3rd generation IPad Air can be updated to the current version of iPadOS 14

If your iPad is on iOS12 (as suggested in your profile) then it is not an iPad Air 4. The Air 4 shipped with iOS14, which was the current OS at the time it was launched.

iOS13 is not available for download at all now - all devices that could run 13 would now update to 14. Best check what model you actually have.