You cannot install Viber 18.x or even Viber 10.x on iOS 4.2.1. The last compatible version was released in 2012-2013 .
| Solution | Feasibility | |----------|--------------| | Use for actual communication. | ✅ Recommended | | Run a local proxy to emulate old API (advanced, unreliable). | ⚠️ Proof-of-concept only | | Switch to Jabber/XMPP with VoIP plugin on iOS 4.2.1 (e.g., Fring .ipa – also dead). | ❌ Most legacy VoIP apps are offline. | Viber .ipa for ios 4.2.1