A new carbon-chain free radical, HC4O, has been identified in the gas phase through its rotational spectrum. The molecule has been generated in a supersonic free jet by electric discharge of an acetylene/oxygen mixture diluted in Ar, and detected by a Fabry-Perot type Fourier-transform microwave spectrometer. Four harmonically related rotational transitions have been observed in the 9-23 GHz region. They all showed fine structure splittings accompanied with well resolved hyperfine structures due to the hydrogen nucleus. Observation of other isotopomers, DC4O and (HC4O)-O-18, has yielded confirmation for the molecular species. The observed spectra have been well characterized as K-a = 0 transitions of a slightly asymmetric top molecule in a doubler vibronic state. The effective centrifugal distortion constant has been found to be fairly large, implying that the molecule has a bent carbon-chain framework. The present observation indicates that the radical is subjected to a strong (i.e., static) Renner-Teller interaction, resulting in a complete quenching of the orbital angular momentum in a nonlinear molecular structure as is the case of shorter members of the HC,O series. The isotropic hyperfine coupling constant shows that almost half of the unpaired electron lies on the carbon atom adjacent to the hydrogen.