Open Cydia and install . This allows the device to recognize and run unsigned or older .ipa files. 2. Obtain the Legacy IPA
Search for "iOS 4.2.1 IPA collection."
The Viber .ipa file for iOS 4.2.1 was compatible with devices running this version of the operating system, including: Viber .ipa for ios 4.2.1
Are you trying to or use it as a daily driver ? Open Cydia and install
to your device using a tool like iFunBox or a similar file manager. Viber .ipa for ios 4.2.1
If you still want to try, you generally cannot use the standard App Store. Users typically rely on these "grey-market" methods: How to install Viber on an iPhone 3G 4.2.1 firmware