Seems as if the battery has finally failed and died.
Batteries DO NOT last forever.
With a dead battery, nothing can be done with this iPad, now.
If you never made any data backups of this iPad, all of this time, then there is nothing more to be done.
That iPad model is 9-years old and is NOT worth anything.
Putting ANY money into this old a device is a waste of money as this is a fully unsupported and obsolete device, today.
Sorry.
Apple ended support for all 8-10-year old iOS devices nearly 5 years, ago, in 2016.
Third party app developers abandoned and stopped supporting these 8-10-year old iOS devices nearly 3 years, ago, in 2018.
There are, virtually, NO MORE third party apps available from the iOS App Store, any longer!
Third party compatible apps for such old devices are few and far between, now, in 2021.
This includes apps for both iOS 9 AND iOS 10 devices!
Simply put, a 4th gen iPad is too old and obsolete now.
Sorry, but this is the reality for ALL 8-10-year old AND OLDER iOS devices now. Especially now in 2021!
If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.
There are PLENTY of less expensive, newer, older iPad models that can run the latest, current versions of iPadOS.
If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2014 and later.
All of these older model iPads below, CAN run the latest iPadOS versions and the up and coming iPadOS 15.
2014, 9,7 inch screen iPad Air 2 models
2015, 7.9 inch screen iPad Mini 4 models
2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )
2016, 9.7 inch screen iPad Pro models ( 1st gen Pencil support )
2017,10.5 inch screen iPad iPad Pro models ( 1st gen Pencil support )
2017, 9.7 inch screen iPad 5th gen models
2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )
2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )
2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )
Get the largest internal data storage model you can afford as NO Apple iOS device has expandable storage.
I am sorry that you had to learn all of this, but this does not change any of the facts regarding these very old iOS devices.
Sorry & Best of Luck to You!