HP ProBook User Guide - Windows XP

Switching the screen image
Press fn+f4 to switch the screen image among display devices connected to the system. For example,
if a monitor is connected to the computer, pressing fn+f4 alternates the screen image from computer
display to monitor display to simultaneous display on both the computer and the monitor.
Most external monitors receive video information from the computer using the external VGA video
standard. The fn+f4 hotkey can also alternate images among other devices receiving video information
from the computer.
The following video transmission types, with examples of devices that use them, are supported by the
fn+f4 hotkey:
LCD (computer display)
External VGA (most external monitors)
Using the keyboard 43