I used DK weight cotton blend (KnitPick's Twist). I cast on 102 for DD, and she wears a 4T, 5 at the most right now. There's room for her to grow too, but the ribbing at the top, plus the straps, keep it from looking too loose. I would suggest, with the DK weight, to go with either 100 or 104. Because I ended my rounds with K2 and the rounds started with K2, so it makes a K4 just at the join. Not a big deal, but still. Oh, and I used size 6 and 10 needles rather than 7 and 11, but that was just me winging it LOL
For the st count on worsted weight, I don't think you would have to change it, or not by much, because the rib st is stretchy and she says it's for size "4/5ish". What I did with mine, since my st count was a guesstimate, was after the ribbing, I had dd try it on it wasn't too snug nor too loose, so I kept on.