Historically, Viber was compatible with iOS 3.0 and later, and specialized versions were developed for early firmware.
Installing a raw .ipa file on a device running iOS 4.2.1 typically requires bypassing Apple's signature checks through . Viber .ipa for ios 4.2.1
: Often cited as the last stable version for iOS 4.2.1. Historically, Viber was compatible with iOS 3