Google has disabled installing apps from unknown sources and lets you just install apps from Google Play. But to install apps from unknown sources on Nexus4 follow the instruction:
1. Slide down the notification panel with 2 fingers and tap "Settings"
2. Go to "Security"
3. Mark "Unknown Sources" to allow app installation from other sources
4. It is recommended to mark "Verify Apps" too, which disallow or want before installation of apps that may cause harm