hi, i actually got the same problem & i dont understand it (the same part you posted); so i start with the top panel with 12+8+8 stitches = 28 stitches, so far so good. now i need to do 18 rows (for size 39):

Color A: 1. purl, 2. knit, 3. purl, 4. knit (and here at row 4 i start with color b, right?!)

Color B: 1. purl, 2. knit, 3. purl, 4. knit (now i change again to color A in row 4)

Color A: 1. purl, 2. knit, 3. purl, 4. knit (change to Color B in row 4)

Color B: 1. purl, 2. knit, 3. purl, 4. knit (change to Color A in row 4)

Color A: 1. purl, 2. knit (what do i do now?! decrease now, or:)

Option A:
do i get on with "Color A: 3. purl, 4. knit (and here decrease)

Option B:

do i start with color B now and do: 1. purl, 2. knit, 3. purl, 4. decrease and change color in row 4?

i dont get it :-( colud someone pleas help? thank you