UYou can implement a MODBUS protocol in RTU mode with RS485 interface which works fine on PIC(Not that sure about your PIC). U
You can have a multidrop line to rest your different microcontroller devices anand have a nice communication between them on the seriarserial UART.
UYou can make your PIC18F2550 as a master which will initiate the transaction from any other 4 controllers.