Several ways to do that, none of them very quick. Once you get it setup you can check the "Keep Outputs" box on the SSD4 mixer page.
In the example I uploaded (SPlat, but is same in X3), I inserted SSD4 into the Synth Rack and chose "All Synth Audio Outputs: Mono"
This should add 24 mono tracks routed the outputs from SSD4. This is where it can get a little confusing.
The 1st 16 tracks are actually L/R for the 8 stereo outs of the SSD4 and the 8 mono outputs will be routed to tracks 17-24.
Another way is to get your SSD4 internal routing done first, and then assign track inputs per track in Sonar.
Use the SSD4 mixer page to assign individual pieces to the outputs (see image).
If you have a mono kitpiece and only a stereo output available, using the pan control on the SSD4 mixer to route the signal 100% L or R. You can then easily pan the piece as needed in Sonar.
Hope that helps. Good Luck!