We report the 364-nm photoelectron spectrum of HC4N−. We observe electron photodetachment from the bent $\tilde X$X̃2A″ state of HC4N− to both the near-linear $\tilde X$X̃3A″ and the bent ã 1A′ states of neutral HC4N. We observe an extended, unresolved vibrational progression corresponding to $\tilde X$X̃3A″ ← $\tilde X$X̃2A″ photodetachment, and we measure the electron affinity (EA) of the $\tilde X$X̃3A″ state of HC4N to be 2.05(8) eV. Photodetachment to the bent ã 1A′ state results in a single intense origin peak at a binding energy of 2.809(4) eV, from which we determine the singlet-triplet splitting (ΔEST) of HC4N: 0.76(8) eV. For comparison and to aid in the interpretation of the HC4N− spectrum, we also report the 364-nm photoelectron spectra of HCCN− and DCCN−. Improved signal-to-noise over the previous HCCN− and DCCN− photoelectron spectra allows for a more precise determination of the EAs and ΔESTs of HCCN and DCCN. The EAs of HCCN and DCCN are measured to be 2.001(15) eV and 1.998(15) eV, respectively; ΔEST(HCCN) is 0.510(15) eV and ΔEST(DCCN) is 0.508(15) eV. These results are discussed in the context of other organic carbene chains.