We present the first infrared ellipsoidal study of a low-mass X-ray binary system, and hence measure the mass of the neutron star. Photometry of Cen X-4 obtained during 1990-91 allows us to determine the quiescent light curve. By fitting this light curve with an ellipsoidal model we revise the ephemeris, obtain solutions in the mass ratio inclination plane, and find that, for reasonable values of the mass ratio, the system is at a low inclination, 31°-54° (90 per cent confidence). From spectroscopic data we find that the contamination of the light curve due to the accretion disc in the infrared is probably less than 10 per cent. Furthermore, by combining the mass function with the above solutions we show that the mass of e compact object lies in the range 0.5-2.1 Msun (90 per cent confidence), which is consistent with the mass ranges predicted by theory for neutron stars. We also show that the secondary of Cen X-4 is a subgiant which could be the progenitor of the stripped giant secondaries observed in systems with orbital periods greater than 1 d.