Because Facebook changed its server-side API, it is highly likely that even if you find the .ipa file, the app will launch but fail to log in, showing a "network error" or "cannot connect" message. Steps to Find and Install Messenger on iOS 4.2.1
Downloading unsigned IPAs from third-party websites is risky. Many legacy app repositories have been compromised. Here’s what to look for: facebook messenger ipa for ios 4.2.1 download
If you still wish to attempt an installation for archival or collection purposes, these are the methods historically used by the Legacy Jailbreak community: Because Facebook changed its server-side API, it is
"SSL Error: Cannot verify server identity" or "Network connection lost." Here’s what to look for: If you still
Even if you successfully install a legacy version of Facebook Messenger on iOS 4.2.1,
If you previously downloaded Messenger on your Apple ID when you had a newer iOS device: