On a multiple of 4+1 stitches
Row 1 (RS): P1, *K3, P1 repeat from * to end of row
Row 2: K2, P1 *K3, P1 repeat until last 2 sts, K2
NOTE: be sure to mark the right side, as this rib is really totally reversible and you might get confused!!
It has turned out to be wonderful in any gauge with any size yarn or needle - truly reversible!