The four main wavelengths used in fiber optic systems are: 850 nm: Primarily used in multimode short-range applications like building networks and data centers due to its low cost and readily available LEDs. 1310 nm: Commonly used in long-haul single-mode applications due to its lower attenuation compared to 850 nm and good availability of laser sources. 1550 nm: The "sweet spot" for long-haul, high-bandwidth single-mode transmission due to its minimal attenuation and mature technology. 1490 nm: Used in certain Passive Optical Networks (PONs) for downstream data transmission due to its low cost and compatibility with existing equipment.