Is that the sound of goalpost moving that I can hear ?
PC to master Arduino via serial link is easy. See Serial input basics - updated
The data sent will include a means of identifying the target Arduino which will be one of 4.
Parse the data from the PC on the master Arduino and send it to the appropriate slave based on the destination decoded from the PC message. How you pass it on depends on you. A serial interface is possible but will mean that the master Arduino needs 5 serial interfaces so probably not the best solution.
So, how about using I2C to distribute the data ? Put each slave Arduino on a common bus, give them each a different address and send the data as appropriate.