Casting off - shoulder shaping

i’m doing the two front sides for Rowan Colouscape Folk - Polly and it says that for the second side i should reverse shapings. it woked fine until i got to the point here iwas sposed to cast off 4 sts and then continue K to the end. when casting off was at the beg of knit row -no problem, but for the right front side, when reversed, it comes up at the END of Krow and then asks you to purl next row and continue. how do i do that?! do i cast off 4 at the beginning of Prow instead?

Exactly. Cast off 4 at the beginning of the purl row and then purl to the end of the row.