Earlier today, a pre-production Android 5.0 firmware from the Nexus 6 leaked on the Internet, which was then quickly ported to the Nexus 4. Unlike the updated developer preview images for the Nexus 5 and 7 though, this leaked firmware came with new versions of Google Apps that featured a Material design. 

However, since the APKs of these apps were signed with test-keys, it was not possible to install them on any other Android device. Thankfully, XDA developer Omar1c has managed to port all the new Material Design apps like Play Music, Play Books, Messenger, Google Fit, Google Calendar and more to all Android devices running Android 4.4 KitKat.

There is a catch here though. The APKs are available in a flashable ZIP form, which means you will need root access along with a custom recovery to install them. If you fulfil this requirement, download the ZIP file from here.

If you face any issues with the app after installing them, especially Hangouts, head over to the system/app/ folder and manually delete their APKs. This will allow you to install the stable version of the app from the Play Store.