Way back when you started the heel flap you worked across 18sts and left the other 18sts unworked (36sts cast on).
Work across 16 (18, 20) sts. Leave remaining sts
That's the 18sts that you're now going to include in working the sock in the round again. You should finish the heel with 12sts on the needle, pick up 8sts down one side, knit the 18sts you left unworked when you started the heel, pick up 8sts on the other side of the heel flap (46sts). I'm not sure where your 4sts are coming from?
See if this tutorial helps orient you in the sock: http://www.cometosilver.com/socks/