This is one of the earlier generation of 10gigabit ethernet fibre optic transceivers. This was designed to use the cheaper multi-mode optical pathway, in order to provide a retrofit upgrade for existing 1 gigabit systems without the need to replace the fibre.
Multimode fibre which this was designed for, causes some time blurring of the pulses (as some rays (modes) of light take longer to arrive than others because they take a longer path through the fibre). This made use of 10 Gbps signalling over old-standard multimode fibre impractical over anything other than a very short distance. The LX4 standard gets round this by using 4 wavelengths, each operating at at about 3 Gbps.
The Xenpak system was also designed with 4 electrical parallel signal paths, in order to make the electrical system simpler.
Modern "laser optimised" multimode fibre is built to higher tolerances so can give acceptable distances using a single wavelength at 10 Gbps. Similarly, modern SFP+ transceivers use 10 Gbps electrical signalling, allowing a more compact and cheaper device.
The Laser multiplexer uses a set of 4 dichroic mirrors to combine the individual laser beams. Presumably, the optical demultiplexer uses a similar technique.