Android 5.1 Lollipop isn't that surprising but whatever software update comes, better update it. After releasing the Factory image for Nexus 5, Nexus 6 and Nexus 10, the Over the Air rollout are happening one by one. This time is the the Nexus 10 which gets the Android 5.1 Lollipop update via OTA download.
The update is happen to rollout in phases and you should charge your tablet to atleast 50% battery life to receive this OTA update.
Nexus 4 is yet to receive either the factory image or the OTA update. If you wanna check what device will receive the 5.1 Lollipop update, then checkout Android 5.1 Lollipop tracker.