40GBASE-SR4 and 40GBASE-LR4 are both types of Ethernet technology but have key differences in their specifications and use cases.

40GBASE-SR4 (Short Range) is a multi-mode fiber optic solution designed for short-range transmission, typically within a data center or other high-density network environments. It operates over 4 separate lanes of multi-mode fiber, and has a maximum reach of 100 meters.

40GBASE-LR4 (Long Range) is a single-mode fiber optic solution designed for longer-range transmission, typically for connections between buildings or across larger campus environments. It also operates over 4 separate lanes, but uses single-mode fiber, and has a maximum reach of 10 kilometers.

In summary, the main difference between 40GBASE-SR4 and 40GBASE-LR4 is the type of fiber optic cable they are compatible with and the maximum transmission distance they support.