LR4 (Long Range 4) and FR4 (Fiber Range 4) refer to different types of optical transceivers used in high-speed data communication over fiber optic links. These modules are typically used for 100 Gigabit Ethernet (100GbE) applications and are designed for longer-distance transmissions. Here are the key differences between LR4 and FR4: 

LR4 (Long Range 4):

1. Wavelengths: LR4 transceivers use four separate wavelengths (usually in the 1264.5 nm to 1337.5 nm range) for bidirectional communication over a single-mode fiber (SMF).

2. Reach: LR4 modules are designed for longer-range transmissions and typically support distances up to 10 kilometers (km) over SMF.

3. Application: LR4 is commonly used in scenarios where the network requires longer-distance connections, such as in metro and regional networks.

FR4 (Fiber Range 4):

1. Wavelengths: FR4 transceivers also use four separate wavelengths, but the specific wavelengths may vary depending on the manufacturer and implementation.

2. Reach: FR4 modules are designed for extended-range transmissions, providing longer reach compared to LR4. They can support distances beyond 10 km, often up to 25 km or more over SMF.

3. Application: FR4 is used in scenarios where even longer-distance connections are required, such as in long-haul and inter-city networks.

In summary, the main difference between LR4 and FR4 is that LR4 is specifically designed for high-frequency and high-speed applications with low signal loss, while FR4 is a general-purpose laminate material for a wide range of circuit board applications.