how to ask google to repair nexus 4?
Cracking your screen most likely isn't covered through a warranty. This is a good reason why you should purchase a screen protector. Mime Ghost Glass would be my recommendation as far as screen protectors go...

You can try to email Google's support: googleplay-support@google.com, explain what happened and you can probably pay to get it repaired, however the Nexus 5 should be coming out in the very near future for $350 so it may be better to just purchase a new device for a little more as screen repairs are usually $100+