The infrared absorption spectrum of the linear C-4 radical has been studied in an extension of the original observation of gas-phase C-4 by Heath and Saykally [J. Chem. Phys. 94, 3271 (1991)]. The experiment was performed using a flowing mixture of acetylene and helium subjected to a hollow-cathode discharge, which was probed in the 1525-1570 cm(-1) spectral region using a tunable diode laser spectrometer. Transitions with N-values up to 60 were measured. Their analysis yielded band origins, rotational, and centrifugal distortion parameters for the lower and upper vibrational states, and I-type doubling parameters for the degenerate bending states v(5) and v(3)+v(5). In particular, the v(3) Origin was determined to be 1548.6128(4) cm(-1), the ground state rotational and centrifugal distortion parameters were B=4979.89(21) MHz and D=0.848(44) kHz, and the l-doubling parameters for v(5) was q(5)=10.98(13) MHz. This value for q, was used to estimate the v(5) frequency of gas-phase C-4 to be 160+/-4 cm(-1). Both the l=0 and 2 components of the v(3)+2v(5)-2v(5) sequence band were also tentatively observed, but a detailed analysis was not yet possible. The results were completely consistent with a linear structure for the triplet ground state of C-4, and showed no effects of quasilinearity such as that exhibited by C-3