OM4 (Optical Multimode 4) is a type of multimode fiber defined by the G.651.1 standard. It has a core diameter of 50 microns and is designed for high-bandwidth applications over longer distances than OM3, supporting up to 10 Gbps over distances of up to 550 meters and 40 Gbps over distances of up to 150 meters. OM4 is commonly used for data center and enterprise networks and is optimized for use with VCSEL sources, offering lower attenuation and higher bandwidth than OM3.