There is a minor security update being pushed out now. The next OS update will be Android 5... Key Lime Pie. Rumors have that update coming out in October. The Nexus 4 will receive it shortly after it's formally released.
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Like the post above, the next major update will be 5.0. Rumors are flying about the net as to when. If not in the very near future, My guess will be next summer. (Only a guess, don't hold me to it...lol)