Assalamoalaikum Zafar! To me it seems like you want to make a serial link between 5 devices. 4 of them are microcontrollers and 1 is your PC serial port.

I don't think the PC serial port has enough juice to drive 4 other ports. (I may be wrong. Please find that out)

There is one more thing. How many devices will transmit and how many will receive? PC serial port does not have the capability (neither does uC serial port) to float its pins while another device (1 of the 4 transmitting devices for e.g.)is transmitting.

If you want to make PC the transmitter and uCs as the receiver...ummm......I think you will have to use some sort of buffer/driver that can feed 4 serial ports (uC). MAX232 will translate the logic levels for the microcontroller. Read its data sheet for more info.

I suggest you make an RS485 network. That way you can have 32 devices on the same bus! Where all can transmit and receive as well.
Search for RS485 chip.

Regards
Aman