We demonstrate a broadband and continuously tunable 1×4 optical beamforming network (OBFN), based on the hybrid integration of indium phosphide (InP) components in the silicon nitride (Si3N4) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si3N4 external cavity laser, a pair of InP phase modulators, a Si3N4 optical single-sideband full carrier (SSBFC) filter followed by four tunable optical true time delay lines (OTTDLs), and four InP photodetectors. The performance of the OBFN-PIC is experimentally characterized by measuring the link gain, noise figure, and spurious free dynamic range of the microwave photonics links. Moreover, we assess its beamforming capabilities assuming that the OBFN-PIC is part of a wireless system operating in the downlink direction and feeds a multielement antenna array. Using microwave signals at 5 and 10 GHz with quadrature amplitude modulation (QAM) formats at 500 Mbaud, we evaluate the performance of the OBFN-PIC under various configurations. An error-free performance is achieved for all the experimental cases validating the potential of the proposed OBFN-PIC for high-quality beamforming performance. To our best of knowledge, this is the first thorough performance evaluation of a fully integrated OBFN-PIC.