swatch! cast on 20 stitches and see what you get!
and if you want a 4 inch wide scarf, and a pattern that is 12 +6, (a minimum of 18 stitches to have a pattern) you either want a 8 inch scarf, a different yarn/gauge, or a different pattern!
you can't knit a 12(+6) stitch pattern into 4 inch scarf if you get a gauge of 10 stitches =4 inches!
this is the designer cunnundrum! finding the right pattern, at the right gauge for the right yarn!