SFP cages usually mean fiber, but sometimes what you actually need is one more copper port. The PR4JR20K answers that case: a hot-pluggable SFP module with an RJ45 jack on the front, converting the SFP slot on a compatible Teltonika router or switch into a standard Gigabit Ethernet copper interface. The module operates at the SFP-standard 1.25 Gb/s line rate, presenting a 1000BASE-T port that runs over ordinary Cat5e or better twisted-pair cabling with the standard 100 m reach. Practically, that turns a fiber-ready device into a more flexible one. A TSW switch with its copper ports full can absorb one more drop. A RUTX router whose SFP cage sits empty because the site never pulled fiber gains a usable uplink or LAN port. Integrators can stock one Teltonika device configuration and decide per site — fiber module or copper module — at installation time. Because the module is hot-pluggable, the conversion happens without downtime: slide it into the cage, patch in the RJ45, and the port negotiates like any Gigabit copper interface. Swapping back to fiber later is equally non-disruptive. Copper SFPs do run warmer than optical modules — normal for the breed, as the 1000BASE-T PHY works harder than a short-reach laser. Give cabinet airflow the usual consideration in hot enclosures. For fiber needs, the companion PR4DU22K (multimode SR) and PR4LC21K (20 km BiDi) modules cover the optical side of the family.