iPhone 4s older version app support

Is your iPhone updated to the latest supported iOS? At a certain point, perhaps when an app's publisher changes, the older versions aren't going to be available on iTunes anymore. The only way you're going to be able to install an older version that's not available from the iTunes servers is to find a copy of the app that you downloaded using your Apple ID, possibly in your iTunes Library (copies of iOS apps weren't deleted from iTunes Libraries even though iTunes lost the ability to sync and manage them partly for this reason) or find a copy with the DRM removed, and go through some unusual steps to load it onto your phone. I don't think in this case the removal of DRM raises any substantial issue with regards to piracy since the app is free, but no longer have a way to verify the integrity of such an app.

Try digging in your iTunes Library or their backups for .ipa files that are left over.

depending on the developer it may be impossible but try reading

https://appleinsider.com/articles/19/03/10/how-to-download-prior-versions-of-apps-onto-an-older-iphone-or-ipad-that-cant-run-ios-12