Rado, adding the -hupcl option on the stty command for the the Arduino port worked like a charm. I can now use echo to communicate with the Arduino.

Code:
stty -F /dev/ttyUSB3 cs8 9600 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts
Thanks, Norbert