Q: How do I install viber app in my iphone 4?

From what I understand, Viber requires a higher iOS than you can use on the iPhone 4. According to the information in the US App Store, it requires iOS 8 or better, and the iPhone 4 will only support up to iOS 7.1.2. You can check this support document, but the developer determines if older versions are still available. Install an app on an earlier version of iOS or OS X - Apple Support