It uses four lanes of multi-mode fiber delivering serialized data at a rate of 10.3125 Gbit/s per lane. 40GBASE-SR4 has a reach of 100 m on OM3 and 150m on OM4. There is a longer range variant 40G-eSR4 with a reach of 300 m on OM3 and 400 m on OM4. This extended reach is equivalent to the reach of 10G-SR.
The QSFP 40G-SR4 module supports link lengths of 100 meters and 150 meters, respectively, on laser-optimized OM3 and OM4 multi-mode fibers. It requires special MTP/MPO cabling. And MTP/MPO fiber cable is a ribbon band that bundles together usually 12 fiber strands.