Once you have the file and a jailbroken device, you have a few ways to sideload it: Cydia Impactor (Legacy) : A classic tool for dragging and dropping files from your computer to your iPhone. iTools or 3uTools
Apple has always been strict about installation. Unless an app is from the App Store, the device refuses to run it. On iOS 4.2.1, "Sideloading" (installing apps manually) was possible but complex. Viber .ipa for ios 4.2.1