QSFP28 and CFP4 are both types of high-speed transceivers used in data communication applications, such as networking and telecommunications. The main difference between QSFP28 and CFP4 lies in their form factor and intended use.

QSFP28 (Quad Small Form-factor Pluggable 28) is a compact, hot-pluggable transceiver designed for high-speed data transmission over multi-mode and single-mode fiber optic cables. It is commonly used in data centers and high-performance computing environments and can support data rates of up to 100 Gbps. QSFP28 transceivers are smaller in size and have a lower power consumption compared to previous generations of QSFP transceivers.

CFP4 (C Form-factor Pluggable 4) is also a high-speed transceiver, but it is larger in size and designed for longer reach applications, such as metro and long-haul networks. CFP4 transceivers can support data rates of up to 100 Gbps and are typically used in network equipment that requires higher power and longer distance transmission capabilities.

In summary, the main difference between QSFP28 and CFP4 lies in their form factor and intended use, with QSFP28 being smaller and more suitable for shorter-reach applications, while CFP4 is larger and designed for longer reach and higher power applications.