An experimental intensity interferometer has been constructed with two searchlight mirrors and tested on Sirius. The correlation observed with the two mirrors close together was found to be in good agreement with that expected theoretically. This result supports the prediction, made in part III of the present series, that the performance of an intensity interferometer should not be significantly affected by atmospheric scintillation. Observations of Sirius were carried out with four different baselines and the decrease of correlation with increasing baseline length was found to be consistent with theory. The observed results have been used to derive an experimental value for the angular diameter of Sirius which is in good agreement with the value given by astrophysical theory. The results of this preliminary experiment confirm, to a considerable extent, the general conclusions reached in part III.