Viber currently requires iOS 6 to run the latest version of the app. Maybe they've dropped support for the older versions of their app, which would explain why it won't run on your phone.
You could restore your 4S and update to the latest iOS, then you could use the new version of the Viber app. The Pangu jailbreak works in iOS 7.1.2 so getting your jailbreak back would be no problem.
One word of warning, though. Although Viber doesn't block jailbroken devices from using Viber they don't support the practice either, so don't expect any help from them if the app has problems with the jailbreak.