iPad Air 4 - Sudden Green Line on Display

Expanding upon Lobsterghost1’s reply, a reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...

When reviewed, if the coloured line is not present in the screenshot image, then you have demonstrated that you do have a hardware fault that, sooner or later, may become sufficiently severe that repair or replacement of the iPad are the only options. If the line is visible in the screenshot image, then something other than hardware will be the cause.

Note: When reviewing the image, rotate the iPad screen orientation by 90°, from Portrait to Landscape (or voce versa), as this will ensure that the image does not coincide with screen defects if present.

Take a screenshot on your iPad – Apple Support

If a hardware fault is confirmed - and you cannot live with the problem - your iPad will require repair:

iPad Repair - Official Apple Support

If you have benefit of an active AppleCare+ plan, you will only pay a nominal excess/deductible fee - otherwise the repair will be expensive, typically 60% of the “as new” cost of the iPad.

It sounds like a hardware issue. Schedule an appointment at your Apple Store Genius Bar for evaluation and options.

A much better expansion of my response! ;-)