Finding a working Viber for iOS 4.2.1 is difficult because Rakuten Viber no longer supports such old versions of iOS
Search for "iOS 4.2.1 IPA collection."
Even if you install the .ipa, Viber’s servers may no longer support the old authentication protocols used by version 2.x. Viber .ipa for ios 4.2.1
Before you begin, make sure you have the following: Finding a working Viber for iOS 4