Using my iPad 4th generation as a kindle reader

The iPad4 is a 32-bit architecture device - that is long ago obsolete. Be aware that all compatible 32-bit Apps have been retired and withdrawn from the Apple App Store. As such, there are now no compatible Apps for your iPad4.

Any Apps already present on your iPad cannot be reinstalled.

As the current Kindle App requires an iPad to have iPadOS14 or later installed, you have no alternative other than to consider replacement of your iPad4 with a much newer or current model. Of current models, even the entry-level iPad9 has specifications and capabilities that will massively outperform your current iPad - supports iPadOS17 - and is compatible with the vast majority of current Apps. Other models are more powerful still.

Download a compatible version of the Kindle App to it, then transfer any ebooks or amazon purchased books to it as described in the kindle apps's user guide.

https://www.amazon.com/gp/help/customer/display.html?nodeId=GD2V96KJZJ6PL3FL

The current Kindle App requires iOS 14, so cannot run on an iPad running iOS 10. But there may be a compatible version you can download from your Purchase history, if you have already donwloaded the App onto a different device using the same Apple ID.

click here ➜ Redownload apps and games from Apple - Apple Support

No kindle is better