Avoid generic download blogs. Instead, use community-driven preservation projects:
Since you cannot download it from the official App Store (Apple will throw error 2807 or suggest updating iOS), you must rely on third-party archives. Here are the top three sources for legacy IPAs: facebook messenger ipa for ios 421 download top
Since the official App Store only provides the latest versions, users rely on digital archives for historical IPA files. Avoid generic download blogs
While not officially for iOS 4, some modded "Lite" versions can work better on older hardware. If you are interested, I can also: Avoid generic download blogs. Instead
: If you use the Whited00r custom firmware, utilize their built-in community app store built specifically for armv6 and armv7 legacy devices. Step-by-Step Installation Guide