Only two telephone wires are used for each phone line, so a four-wire phone jack is able to provide service for two lines. These phone jacks typically have two ports for plugging in phone cords. A telephone and dedicated fax line is one option for the four-wire phone jack. Arrange the telephone wiring in a dual port jack to service the needs of your home.
Slice the cable jacket on the outside of an incoming phone line lengthwise with a utility knife. Make a 10-inch slit in the cable and cut away all 10 inches of the jacket to free the wires within. A four-wire phone line has the old standard red, green, black and yellow wires. Newer lines may have eight wires.
Separate the correct four coloured wires from newer phone lines for connection to the jack. The primary four wires connected to the telephone interface by the phone company are orange with white, white with orange, blue with white and white with blue. Bundle the other four wires together and secure them with a twist tie to keep them apart from the wires to be used on the four-wire jack.
Cut a 5-inch length from each of the four wires. Strip one end of each cut length with wire cutters to reveal 3/4 inch of bare wire. Strip the other end to expose 1/2 inch of bare wire. Strip the ends of the four incoming phone wires to expose 1/2 inch of bare wire as well.
Loosen the four terminals inside the jack housing. Wrap the four incoming phone lines around the terminals marked with the first letters of the old telephone wire colours--R, G, B and Y. Connect old colour wires to the corresponding coloured terminal wire.
Wire the new colours to the terminals marked with the older colour codes if necessary. Wrap the white wire with blue marks around the G terminal. Put the blue wire with white marks on the R terminal. Put the white wire with orange markings around the B terminal, and the orange wire with white marks on the Y terminal. Wrap the bare wire completely around the terminal with clockwise turns.
Connect the short wire lengths to the terminals. Retain the colour codes for ease of identification. Wrap the ends with 1/2 inch of exposed wire around the terminals. Tighten each terminal with a Phillips screwdriver.
Connect the other ends of the short lengths to the jack ports. Use the wires from the R and G terminal for one port, and the wires from the B and Y terminals for the other. Push the 3/4-inch exposed wires into the two holes at the back of the jack socket that are in the centre. Push the wires all the way in and bend them into the two central wire canals where the phone plug enters.
Push the jack sockets into the socket cups with the openings out for receiving phone plugs. Gently push the jack housing over the terminal plate and tighten the holding screw with a Phillips screwdriver. Plug in a telephone or fax machine and test the lines.