400GBASE-XDR4 “eXtended reach DR4” transceiver, up to 2km over parallel SMF. The 400G-XDR4 can break out to 4 x 100G, and interop with 4 x 100G-FR QSFPs.
2km reach using single-mode fiber, and “4” implies there are 4 optical channels. Unlike the DR4 and SR8, all of the 4 optical channels from an FR4 are multiplexed onto one fiber, resulting in a total of 2 fibers from the module (1 Tx and 1 Rx). Each optical channel operates at 100Gb/s. The FR4 module uses a duplex LC connector to connect to a single fiber pair.