I agree with what Mr. Erik Malund has said Data coming from PC serial port is 8 bit with start, stop bits. But if in your case it is 4 bit [ I dont know Why? ].
Put the Tx from PC to any of the port pin and simply wait in a loop for that pin to go low i.e. stop bit, and then start a timer say timer 0 to delay for 7.8125uS and then you will get first bit at port pin and then wait for same delay again and then you will get second bit and so on after stop bit is read again wait in the loop for line to go low i.e. start bit and continue like this.