Using a newly developed dual beam laser plasma method, the C-4(-) anion cluster has been formed an? deposited in an argon matrix and studied via Fourier transform infrared spectroscopy. An infrared band at 1699.8 cm(-1), previously assigned to a combination mode of the neutral C-4 cluster, is shown to be due to the asymmetric stretching fundamental of the C-4(-) anion. This attribution is based on the different behavior of the known 1543.4 cm(-1) neutral C-4 asymmetric stretching mode and the 1699.8 cm(-1) band under action of annealing, photolysis, and addition of the electron trap, CCl4, to the plasma gas. Density functional theory and ab initio theory calculations predict a stable, linear C-4 anionic cluster that should possess a moderately strong IR band close to this position. Previous isotopic (C-12/C-13) Splitting patterns are reinterpreted and shown to match very well with the theoretically predicted splitting patterns for C-4(-) An electron photodetachment energy oi. 4.51 eV is found and compares well with the gas-phase value when account is taken of the matrix stabilization energy of 0.63 eV. It is proposed that anions are formed via electron capture by preformed neutral C-4 clusters and that a steady state of neutral and anionic C-4 clusters is established in the matrix.