Laser-induced fluorescence (LIF) spectra of the C4H radical and its isotopomer C4D have been observed in a supersonic free jet expansion for the first time. The jet cooled free radicals have been generated in an electric discharge of 0.5% C2H2 or C2D2 in Ar using a pulsed discharge nozzle (PDN). Twenty and eighteen vibronic bands of C4H and C4D, respectively, have been observed in the 24 000-25 000 cm(-1) region. Most of the observed bands have been assigned as either (2) Pi-(2) Sigma or two types of (2) Sigma-(2) Sigma. Effective spin-orbit interaction constants were determined by analyses of the (2) Pi-(2) Sigma bands to be -14.7644(80) and -10.9926(35) cm(-1) for C4H and C4D, respectively, leading to conclusion that the upper electronic state of the observed band system is (B) over tilde (2) Pi(i), in agreement with a recent ab initio calculation by Sobolewski and Adamowicz [J. Chem. Phys. 102, 394 (1995)]. Observation of two types of (2) Sigma-(2) Sigma bands, (2) Sigma(+)-(2) Sigma(+) and (2) Sigma((+/-))-(2) Sigma(+), is explained by the difference of the magnitudes of Renner-Teller interactions for the bending vibrational modes involved. We were able to assign the nu(5) (CCH bending) and nu(6) (CCC bending) progressions of the (B) over tilde (2) Pi(i) state, where the Renner-Teller interaction is large for nu(5), and small for nu(6). The vibrational frequencies and the Renner parameters were determined to be omega(5)=344 cm(-1), epsilon(5) = -0.657, omega(6) = 189.3 cm(-1), and epsilon(6) = -0.0158 for C4H, and omega(5) = 295 cm(-1), epsilon(5) = -0.692, omega(6) = 183.4 cm(-1), and epsilon(6) = -0.0188 for C4D, respectively. Intensities of the symmetry forbidden (2) Sigma-(2) Sigma bands were larger than those of the symmetry allowed (2) Pi-(2) Sigma bands, suggesting a very small energy gap and strong vibronic mixing between the (A) over tilde (2) Pi-(2) Sigma and (X) over tilde (2) Sigma(+) states. Fluorescence lifetime profiles exhibited fast decay (10-20 ns) followed by a very weak and slow decay (3-4 mu s) component with complicated beat structures. The implication is that C4H in the (B) over tilde (2) Pi(i) state has a fast relaxation path to nonradiative states through internal conversion, and therefore does not dissociate under near uv radiation at 400-417 nm.