We present results of an ESR study of tetra-interstitial agglomerates (I4) in silicon. The defects were formed upon annealing of electron-irradiated silicon samples pre-doped with hydrogen and reveal two ESR spectra, i.e., the B3 which is related to positive charge state of the defect and the NL51 which is related to the neutral defect in an excitonic triplet state. Temperature dependence of the intensity of the NL51 spectra allowed determining parameters of the excitonic state. From photo-ESR measurements, the energy level of the excitonic state was determined to lie at ∼1.03eV above the valence band.