Your multi-meter is averaging the voltage on the yellow wire. Since it is Pulse Width Modulated (PWM) the voltage varies between approximately 0 volts and approximately 5 volts. Because the voltage is not present during the full cycle your meter averages the voltage.
The PWM duty cycle varies from 0 to 100 percent based on the spindle speed selected by the S word in the G-code.
Since you are running version 0.9j, the max spindle speed is set in the firmware to about 12000 RPM. So to get approximately 5 volts on the yellow wire you would have to have the spindle on and set for 12000 or greater RPM (M3 S12000).
You would get better results if you upgrade grbl to my 1.0c or the official 1.1f
I would recommend my version over 1.1f for laser work unless you need the feed rate and spindle speed over ride features for spindle work.