Cyclobutanetetrone, also called tetraoxocyclobutane, is an organic compound with formula C4O4 or (-(C=O)-)4, the fourfold ketone of cyclobutane. It would be an oxide of carbon, indeed a tetramer of carbon monoxide.
The compound seems to be thermodynamically unstable. As of 2000, it had yet to be synthesized in significant amounts but may have transient existence as detected by mass spectrometry.