Sorry for the very late update.
The fixture, Nowlit - QR-L60, is a PAR with White color in addition to RGB.
rekordbox does not have any function to operate White of any fixtures with RGB. This is why you cannot use Channel 4.
In order to smoothly change the Color transition, when white is assigned, white lights is lit using the mixture of RGB instead of the white color itself.
We are aware of similar requests, but we have not started implementing such a change in this type of fixtures yet.
Thank you for your kind understanding.
Pioneer DJ Support