Below is an excerpt form a conversation I had with Michael Lane about using the doser adaptor board and how he advises setting it up for the fourth channel
The dosing adapter works using the PWM signal from the PCA9685. Electrically, it would work fine with a gpio from Power1, but it would not show up as a doser jack, and would only be fully on or off. Any of the Light ports can be used to control the 4th channel and work fully as a doser in reef-pi.
Although the Doser port only has 3 outputs, you can still use a DP4S by connecting the 4th input to one of the light outputs. I usually recommend using Light 3.
In this case, the white wire is connected to PWM ~7 on Light 3.