40GBase CSR4 and 40GBase SR4 are both standards for 40 Gigabit Ethernet over fiber optic cables, but they have some differences.

40GBase CSR4 (short for 40 Gigabit Ethernet Consortium Specification Revision 4) uses a special type of multi-mode fiber optic cable called OM3 or OM4, and it uses four lanes of 10Gbps signals over the fiber. It has a maximum reach of 300 meters.

40GBase SR4 (short for 40 Gigabit Ethernet Short Range 4) also uses multi-mode fiber optic cables, and it also uses four lanes of 10Gbps signals over the fiber. It has a maximum reach of 100 meters.

The main difference between the two standards is the reach - 40GBase CSR4 has a longer maximum reach of 300 meters compared to 40GBase SR4's 100 meters. Additionally, 40GBase CSR4 is specifically designed for use with OM3 and OM4 fiber optic cables, while 40GBase SR4 can also work with OM2 cables in some cases.