UKHeliBob:
A serial interface is possible but will mean that the master Arduino needs 5 serial interfaces so probably not the best solution.
You can connect a single Master serial port (I am assuming a Mega or Leonardo with an extra HardwareSerial port so as to use Serial1 rather that Serial) to several slaves. If the slave only needs to listen then just connect the master Tx to the Slave RXs. For 2 way communication there needs to be a diode between each slave's Tx and the Master's RX and a pullup resistor for the Master's RX. And, of course it is essential to write all the code so that only one slave talks at any one time.
However I can't help feeling that the overall approach to the project is poorly planned. No evidence has been provided to show that 4 Arduinos offer any advantage to outweigh the inconvenience of communicating with them.
...R