I too am pushing the limits of my 4108's decode ability, so I went to 9 cam mode on the display and set all 6 of my cameras to stream S1 instead of M. Then I went to 4 cam display, came back to 9 cam and the NVR had switched some (but not all) of the cams back to main stream.
Not following any particular pattern, the NVR forgets the stream setting for some cams for 4, 8, and 9 cam display modes.
That's a bug, right?