You have a very old iPad4. Notwithstanding the fact that your available local storage is totally full - your iPad4 cannot be updated beyond iOS 10.3.3/4. Freeing sufficient space on your iPad will very likely require deletion of installed Apps - or to remove cached content, may require restoring the iPad. All of this has implications for ongoing usability.
Deleting email accounts may help - but only to a very limited degree.
Update support for iPad4 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). iPad4 cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions. The 32-bit platform upon which the iPad4 is based is obsolete.
Many App Developers have dropped support for 32-bot devices and iOS versions preceding iOS11/12/13. Withdrawal of Apps supporting older models of iPad is making the utility and usefulness of older iPad models significantly more limited. Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.
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. Of current models, even the entry-level iPad8 supports current iPadOS 14 - and has hardware specifications and capabilities that vastly outperform your old iPad4.
I hope this information is helpful - if only in clearly stating the limitations of your iPad4 and your available options.
