HP Laserjet 4 & Serial Printing

[HPTECH]

How or where do you set it to print Serial.

From Service Manual

Since the default I/O setting on the printer is parallel, remember to change the settings on the printer control panel for serial printing.

[Brian]

As far as I know the HP 4 is auto sensing and therefor doesn't have to be set to either the serial or the parallel interface, you do have to set the serial parameters to suit your serial port.