The rotational spectrum of S-4 has been observed for the first time in an electrical discharge through sulfur vapor. Two techniques have been used: Fourier transform microwave spectroscopy and long-path millimeter-wave absorption spectroscopy. Small, but systematic shifts of the measured transition frequencies of the normal isotopic species indicate that S-4 has C-2v symmetry but with a low-lying transition state of D-2h symmetry, yielding interchange tunneling at 14.1(2) kHz in its ground vibrational state. From the rotational constants of the normal and the single S-34 isotopic species, an experimental (r(0)) structure has been derived: S-4 is a singlet planar trapezoid with a terminal bond length of 1.899(7) Angstrom, a central bond of 2.173(32) Angstrom, and an S-S-S angle of 103.9(8)degrees. Like thiozone (S-3), S-4 is a candidate for detection in the atmosphere of the Jovian moon Io and in other astronomical sources. (C) 2004 American Institute of Physics.