In 2007, the detection of the second interstellar gas phase anion -- C₄H^- -- was reported by Cernicharo et al. in IRC +10216 via observations using the IRAM 30-m telescope. The anion has also been observed in the low-mass star-forming region L1527 by Sakai et al. and by Agúndez et al. High resolution rotational spectra of C₄H^- were issued by McCarthy & Thaddeus and by Amano.

Like the first anion that was detected, C₆H^-, C₄H^- is a linear polyyne species. The anions of C_2nH species are stable because their electron affinities are quite substantial. The measured electron affinity of C₄H reported by Zhou et al. is 3.53 eV, which is comparable to the electron affinities of halogens like F (3.40 eV) and Cl (3.61 eV).