OK, I found the manual for the Elation SDC-6 6 Channel Manual Dmx Controller. It is not particularly clear with respect to DMX, but here is my best guess at what it is saying.
The SDC-6 says it adjusts values of 1~6 channels. I'm guessing that you can't adjust which six channels it controls, i.e., it only sends signals on channels 1 thru 6.
That means you have to set the dip switches on the DP-415 to "listen" on those channels. Note that the DP-415 only handles 4 channels - so two of the slider on the SDC-6 won't cause anything to happen.
The first 9 dip switches on the DP-415 set the DMX "listening" addresses for the unit, and the last switch tells it whether to operate as a switch (on/off) or a dimmer. Whatever you set the dip switches to, the unit will "listen" on that channel and the next three, e.g., if you set it to listen on channel 1, it will listen on channel 1, 2, 3, and 4.
Try setting all the dip switch switches to the down/off position except the first one and the last one. By flipping only the first switch on your are telling it to "listen" on Channel 1, 2, 3 & 4. So it will look like this:
- _ _ _ _ _ _ _ _ -
Now plug each of your lights into its own socket set [I'd recommend that you plug two lights into the left side and two into the right side each as far away from the other as possible - that way you won't risk pugging two lights into the same channel.]
Now sliders 1~4 should brighten and dim lights 1~4. Sliders 5 & 6 won't do anything.