(front) (back)
+-------+ +-------+
-| 4 | 1 | | 3 | 2 |
+| 4 | 1 | | 2 | 3 |
|-------| |-------|
-| 8 | 5 | | 7 | 6 |
+| 8 | 5 | | 6 | 7 |
+-------+ +-------+
To facilitate this layout, --nup4 also pads the input PDF pages to a total number that is a multiple of 8, by adding empty pages if necessary.