Devices running iOS 4.2.1 are easily jailbroken (using tools like Redsn0w or GreenPois0n). Once jailbroken, you can use tools like AppSync Unified (available via the Cydia store) to install unsigned .ipa files. This is the most reliable way to get an old file onto the hardware, but it still doesn't solve the server connectivity issue.
Use tools like on Windows 7 to jailbreak an iPhone 3G running 4.2.1. Viber .ipa for ios 4.2.1
: Legacy versions of Viber lack modern features like communities, disappearing messages, and Face/Touch ID security. Apple Support Community How to Install (Legacy Methods) Devices running iOS 4