The microwave spectrum of 4-isocyano-1-butyne (HC CCH2CH2N C) has been investigated in the 12.4-77.6 GHz spectral region. The spectra of two rotamers denoted ap and sc were assigned. ap has an antiperiplanar arrangement for the C-C-C-N chain of atoms, whereas sc has synclinal conformation for this link. The ground state spectrum and three vibrationally excited state spectra of the lowest torsional vibration were assigned for ap, while the ground vibrational state spectrum was assigned for sc. The C-C-C-N dihedral angle was found to be 64.5(30)degrees in sc and exactly 180 degrees in ap. ap was determined to be 2.9(6) kJ/mol lower in energy than sc from relative intensity measurements. The microwave study has been augmented with ab initio and DFT calculations employing the CCSD(T), MP2, and B3LYP methods with the cc-pVTZ basis set. A Natural Bond Order analysis has also been performed. Most, but not all, of the quantum chemical predictions agree satisfactorily with the experimental results.