Software update

Update support for yoir iPad4 formally ended September 2017. The final IOS releases for this model were iOS 10.3.3 (WiFi Only models) and iOS 10.3.4 (WiFi & Cellular). Your iPad cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS12 - and have withdrawn older versions of their Apps from the Apple App Store.

If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad model supports current iPadOS 13.x - and has hardware specifications that will vastly outperform your old iPad4.

I hope this information is helpful - if only in clearly stating your current options.

You are getting that message because your device is too old to update any further. If you want to run the latest iOS versions, you have to invest in a newer device.