Hello,
Do you want to control each output individually?
May you elaborate on “only channels 3,4 response”? How do you detect the response?
I suspect the operating mode of the device is set by default to RGB+W mode. You may change it by setting the register 0x0FA0 to 512 [register map].
Also, you may need to not only set the channels into on and of states, but also adjust the brightness of each channel using holding registers 0x07D0 to 0x07D3 [in the register map]. It will work with the opetating mode 512.
The device has different modes for different kinds of LED strips, so may be you’ll want to use some of these modes (documentation).
Perhaps we could help you with these operation modes if you describe your task more thoroughly.