The Metro Rail Transit Line 4, also known as MRT Line 4, is an approved mass rapid transit line to be built in Metro Manila and Rizal in the Philippines. When completed, it will be the second rapid transit line serving the province of Rizal after the extension of LRT Line 2 to Antipolo, and is expected to significantly reduce the volume of vehicular traffic along Ortigas Avenue and improve connectivity in the eastern parts of the metropolis, including the nearby municipalities in southwestern Rizal.