The microwave spectrum of 4-isocyano-1-butene (H2C=CHCH2CH2NC) has been investigated in the 35-80 GHz spectral region. Selected measurements have also been made outside this spectral range. Rotation about the -CH-CH2- and -CH2-CH2- single bonds may produce rotational isomerism resulting in five conformers. The spectra of three of them, denoted I, III, and IV, have been assigned. In conformer I, the C=C-C-C link of atoms is +anticlinal and the C-C-C-N chain is antiperiplanar. In III, the two links of atoms are +anticlinal and +synclinal, whereas in IV, the two chains are synperiplanar and antiperiplanar, respectively. Conformer I was found to have the lowest energy of the three forms by relative intensity measurements. These measurements yielded E-III - E-I = 1.1(7) kJ/mol, and E-IV - E-I = 2.9(7) kJ/mol for the internal energy differences. The microwave study was augmented by quantum chemical calculations at the CCSD/cc-pVQZ and MP2/cc-pVTZ levels of theory. Good agreement between experimental and theoretical results was seen in most cases.