Cast on a multiple of 4 sts + 1.
With colour A: purl 2 rows.
Change to colour B (now begins the sequence of rows that form the pattern):
Row 1 (WS): P2, sl1p, *P3, sl1p*, repeat from * to * until the last 2 sts, P2.
Row 2: repeat row 1, slipping the sts with the yarn in the back of the work.
Row 3: repeat row 1.
Row 4: repeat row 2.
Change to colour A
Row 5: P4, sl1p, *P3, sl1p*, repeat from * to * until the last 4 sts, P4.
Row 6: repeat row 5, slipping the sts with the yarn in the back of the work.
Repeat rows 1 to 6.
sl1p: slip 1 stitch purwise