The iPad 4th generation is ineligible and excluded from upgrading to iOS 11
the introduction of iOS 11, ALL support for older 32 bit iDevices and any iOS 32 bit apps has ended.
Your iPad 4 is a 32 bit hardware device.
The new 64 bit coded iOS 11 ONLY supports newer 64 bit hardware iDevices and 64 bit software, now.
The iPad 4 is incompatible with this new iOS, now.
No need to panic or worry though.
Your iPad 4th gen will still work and function as it always has, but will no longer receive any more app updates sometime in the foreseeable future.
The 4th gen iPad has had over 5 years of iOS upgrades/updates
The final app updates your 4th gen iPad will receive will be its last!
Your iPad 4 will survive and stay a viable, functioning iPad for a couple more years.
Currently, iPad 4 models are STILL receiving app updates, but look for this change over time.
This is why Apple introduced the new, lower cost 2017 iPad 5 model, in April
Apple is trying to get older iPad 2, 3, 4 and 1st gen Mini iPad users to upgrade to more modern iPad hardware in anticipation of iOS 11, which will render all older 32 bit iDevices obsolete.
Good Luck to You!