So, I got the OTA update to Kit Kat on my Nexus 4 a few days ago, and I haven't been able to enable the OK Google function from my homescreen. What gives?
Make sure you installed the Google Launcher from the Nexus 5, it's not available in stock Nexus 4 launcher. Go into Google Search's Settings and set default language to English US, also go into Voice -> Offline speech recognition, set English US as default and make sure its updated.