If you are looking to get functional apps on this specific device, please let me know:
You have two paths: (requires iTunes) or with jailbreak (easier).
: Viber officially dropped support for iOS 4.2.1 years ago, stating the minimum requirement was iOS 4.3 or higher. Stability Issues : Even if you find an archived
, the app often crashes or fails to connect to Viber's modern servers, which use updated encryption protocols that older iOS versions can't handle. Feature Loss
If your Apple ID is old enough to have downloaded Viber years ago, you can sometimes trigger a native download: Log into your Apple ID on the iOS 4.2.1 device. Open the App Store and navigate to your tab. Locate Viber and tap download.
Transfer the .ipa file to your device via SSH, a cloud service accessible through Safari, or AirDrop alternatives.
You need a desktop utility capable of communicating with older iOS filesystems, such as Sideloadly, an older version of iTunes (version 12.6.5 or older), or CopyTrans. Where to Find Legacy .ipa Files
: iOS 4.2.1 operates on a strict 32-bit architecture. Modern iOS apps are strictly 64-bit.