using multiple hc-05 with one arduino mega
HC-05 modules use Serial (UART) for communication with the Arduino. Thus you need 4 Serial interfaces for 4 HC-05 modules. The Arduino Mega has 4 hardware Serial interfaces, so just connect once HC-05 to each Serial interface. Then handle each module in your code the same just with a different Serial interface. If you already know, how to use one HC-05, then it should be rather easy to implement the same with all 4 modules.