Whereas 2-Wire smoke detectors use a single pair of wires for both power and sensor circuits, 4-Wire smoke detectors separate the power and the sensor circuit into 2 pairs.
The main downside to 4-wire systems is the ability to monitor system integrity. It is critical that any device removal, wiring damage or power loss result in a fault or trouble condition at the panel. The image below illustrates typical 4-wire detector wiring on a two-piece detector. A two piece detector uses a base with a removable head. The power side wires the same as a 2-wire system. If a head is removed, the power opens.
Not all 4-wire system use a separate base and head. Both positive wires are placed under the same terminal. Removing the head does not open a circuit, removing the wires from the terminals opens the circuit, creating a trouble condition.
The image below shows the wiring for the last detector in a 4-Wire system. However, there is a problem. A 4-wire system is wired Normally Open (NO) on the sensor circuit, no power is required to maintain the NO state. The state of the circuit would remain unchanged from a power loss, cut wire or device removal. If power was required to maintain the NO state then a power loss or cut wire would close the sensor relay and trip a false fire alarm. You don't want a fire alarm every time the power goes out!
In any 4-wire smoke detector system you must install an End-Of-Line power supervision relay. The relay uses the power side of the circuit to maintain a complete sensor circuit. The image below shows the proper wiring for installing an End-Of-Line power supervision relay.
The image below shows a complete circuit after the smoke head is installed. It is important that the resistor is placed in-line of the sensor circuit.
In some systems you might find that the installation technician extended the four wires so that the End-Of-Line power supervision relay is actually located at the panel or a more easily accessible location. There is nothing wrong with this method and in most cases it is better for future service and troubleshooting.