You may harm your Android phone by installing apps from sources other than Play Store. On Samsung Galaxy S4, "Verify Apps" warn or block before installing apps that may cause harm. If you have already let the device install apps from sources other than the Play Store, then we suggest you enable this feature by doing as follows:
1. Open "Settings" and go to "More" tab
2. Tap "Security"
3. Mark "Verify Apps" under "Device Administrations"
4. Note that you can install apps only from Play Store by unmaking the "unknown sources" option
Read More Galaxy S4 Tips and Tricks