Using yarn A cast on 60 sts for cuff and distribute sts over 4 needles, 15 sts on each. The beginning of the round will be between needle I and needle I. Join to work in the round and work 4 cm in rib stitch.
Work 1 round in st st. Next, start following the colourwork chart at row 1 and repeat the 12 stitch pattern for 5 times. Work rows 2–38.
Row 39: Change to yarn A and k to end.
Row 40: K to end and at the same time work k2tog once on each needle = 56 sts.
Continue following the chart at row 41 and repeat the 8 stitch pattern for 7 times. Work rows 42–58.
Change to yarn A and work 4 rows in the following way:
Rows 59–60: K to end.
Row 61: K to end and at the same time work k2tog once on each needle = 52 sts.
Row 62: K to 2 last sts on needle I, k2tog, k to beginning of needle IV, k2tog, k to end = 50 sts.
Reinforced heel flap
Start working the flap by knitting the sts of needle I onto needle IV (= 24 sts). Turn work and leave the sts on the second and third needle on hold for a while. Change to yarn D and start working the reinforced heel flap:
Row 1 (WS): Sl1 wyif, p to end. Turn work.
Row 2 (RS): *Sl1 wyib, k1*, repeat *-* to end. Turn work.
Repeat these 2 rows until you have worked them for 12 times (= 24 rows).
Next row: P to end.
Continue working in the reinforced stitch pattern shown for the heel and at the same time decrease for heel turn:
Next alternate decrease row (RS): Sl1, work to last 9 sts, skp. Turn work.
Next alternate decrease row (WS): Sl1, p6, p2tog. Turn work.
Next alternate decrease row (RS): Sl1, work to last 8 sts, skp. Turn work.
Keep working these decreases on each side of the 8 centre sts until you have only the 8 centre sts left. Now, distribute these sts onto two needles, 4 sts on each.
Using yarn A and with the aid of the fifth needle, pick up 12 sts along the left edge of the heel flap + 1 st between the heel flap and needle II and knit them through back loop onto the left heel flap needle. Knit the sts on needle II and needle III. Pick up 1 st between the needle III and the heel flap + 12 sts along the right edge of the heel flap and knit them through back loop with the aid of the right heel flap needle. Knit the first 4 sts of the heel flap.
Continue with yarn A and work with all 60 sts in st st. Start decreasing for the gusset:
Round 1: K to last 2 sts on needle I, k2tog, work to needle IV, skp, k to end.
Round 2: K to end.
Repeat these 2 rows until you have 12 sts on each needle (= 48 sts).
Keep working straight in st st until the sole measures about 12 cm. After that, start following the chart at row 1 and repeat the 12 stich pattern for 4 times. Work rows 2–19.
Change to yarn B and use it for the rest of the work. Work straight until the sole measures ca. 20 cm or the little toe is covered. Then, start decreasing for the toe:
Round 1: Needle I: K to last 3 sts, k2tog, k1; needle II: k1, skp, k to end; needle III: k to last 3 sts, k2tog, k1; needle IV: k1, skp, k to end.
Round 2: K to end.
Repeat these 2 rows until you have 6 sts on each needle. After that, repeat only row 1 until you have 8 sts left. Cut yarn, thread through the stitches and pull tight. Weave in ends.
Make another sock to match.
Steam block gently.