I can’t download the iOS 14 for my iPad 4

Are you sure that you have an iPad mini4 - and not an earlier model of iPad?

Identify your iPad model.

What is the currently installed version of iOS/iPadOS?

Settings > About

If your installed version is iOS12 or earlier, you may be too far behind for the wireless OTA update from iPad Settings to work. Instead, you’ll need to update your iPad using either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS):

Update your iPhone, iPad or iPod touch – Apple Support

Do you have a beta version of iOS/iPadOS installed? If so, you’ll need to remove the beta profile - as, whist present, the beta profile will block update to the latest production version of iPadOS.

https://beta.apple.com/sp/betaprogram/unenroll#ipados

How to uninstall iOS beta software - Apple Support

Finally, if your iPad still won’t update:

If your iPhone, iPad or iPod touch won’t update - Apple Support