The discovery of the C₄H (butadiynyl) radical toward IRC +10216 was reported in 1978 by Guélin et al., based upon 4 observed doublet rotational transitions that were predicted with molecular structure calculations. The species was subsequently detected in the Taurus Molecular Cloud TMC-1 by Irvine et al. in 1981 and by Bell et al. in 1982. Both of these papers identified new spectroscopic features of C₄H. The rotation spectrum was measured in the lab by Gottlieb et al. in 1983, confirming the earlier theoretical predictions. The spectrum of C₄D was measured by Woodward et al. and then identified in TMC-1 in 1989 by Turner. The C₄H^- anion was identified in 2007.

The butadiynyl radical can be formed by removing one of the H atoms from diacetylene (butadiyne). This suggests a ground state (²Σ⁺) where the singly-occupied orbital is parallel to the molecular axis. While this is indeed the most stable state, there is a ²Π state that lies only about 200 cm^-1 higher in energy (as measured by Zhou et al.). The singly occupied orbitals of each state are shown below.

2Σ+	2Π