Even Nexus devices get end of lifed eventually. The Nexus 4 is an aging device and as such has not been updated past 5.1.1. Who knows if this device will ever see an official build of Android Marshmallow 6.0 from Google. If your phone works and you love it why replace it? Well one good reason to replace it would be the ability to try the latest greatest version of Android.
The great thing about the development community is that developers continue to update devices unofficially long after OEMs update them. The Nexus 4 has just received Marshmallow in the form of the "Frank N4" Rom! This rom is based on stock Android 6.0 AOSP. The rom is rooted, and includes support for Layers. It also includes an advanced reboot menu, volume rocker wake, volume rocker cursor control, long press quick settings control, and this is as close to stock as it gets. Head to the link below for the download.
via XDA