Update Cyanogenmod to latest Snapshot
I too have a Samsung Galaxy Nexus and I upgraded from CM11 M3 to M4 manually just one week ago.

But I looked it up on my phone and there is an OTA-Update to the M4 build.
Other than that, you can find your current CM11-build under Settings -> About phone and there it says in the entry named CyanogenMod-Version. It should end with your device's code-name (depending on your specific Galaxy Nexus it can be either "toro", "toroplus" or "maguro"; in my case "maguro").

And that update in late february cannot have been the M4-build, because that's just too far in the past. The M4-build was anounced officially by the team of CyanogenMod on march 7th.

I hope I could help you.

PS: If you still do not get the OTA-update, I would recommend you to flash it manually.