what is your output of "sudo setserial -av /dev/ttyS4"? if setserial doesn't exist, you can "sudo apt-get install setserial" it.
what is your output of "sudo stty -a -F /dev/ttyS4"?
what is your output of "sudo setserial -av /dev/ttyS4"? if setserial doesn't exist, you can "sudo apt-get install setserial" it.
what is your output of "sudo stty -a -F /dev/ttyS4"?