While it supports Android 4.1+, modern YouTube services strongly require newer Android versions (Android 5.0 and above) for full functionality. Where to Find It
Older APKs do not include recent security patches, which can leave your device vulnerable. API Issues: youtube 11.19.56 apk
Go to your device's Settings > Security and toggle "Unknown Sources" to allow installations from outside the Play Store. While it supports Android 4