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 The 32-bit platform upon which the iPad4 is based is obsolete.
Many App Developers have also dropped support for 32-bit devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store. Withdrawal of Apps supporting older models of iPad is making their utility and usefulness 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 Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.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 iPad9 supports current iPadOS 15 - and has hardware specifications and capabilities that vastly outperform your old iPad4. Other models of iPad are even more powerful.
All computers ultimately have a support lifespan - during which their hardware is sufficiently powerful to be updated with new features and software capabilities. Ultimately, all computers (including iPad) will reach a point where the hardware is technologically too old to support modern software. Apple devices are renowned for receiving support - and remaining very useable - for considerably longer than many other manufacturers; while Apple do not provide a specific forecast, iPad are typically supported for 5 years or longer. If you purchase a current model iPad, you might have reasonable expectation of seeing something similar.
I hope this information is helpful - if only in clearly stating the fundamental limitations of your iPad4 and your available options.