Hi ayersart
If your page size is 5.5" wide and 8.5" tall then you can only get 4 pages on an 8.5" x 11" sheet of paper, therefore it would be multiples of 4.
However if you print on 12" x 17.5 or larger then you could get 8 pages on a sheet, fold it twice then trim to 5.5" x 8.5. Are you going to print this booklet your self or send it to the printer?
Hope this helps
IMacQuarker