Like with Square Three, you will skip stitches and work into the chain-spaces from the second row of your edging.
STEP 1 With a slip knot on your hook, insert into any joined corner space. Chain, and do a double crochet into the same space.
STEP 2 Chain, then do a double crochet into the first stitch on the edge of the square (the one that forms a triangle into the corner-space).
STEP 3 Continue to chain 1, and skip a stitch to double crochet into the next stitch.
STEP 4 When you reach the next corner-space, do one double crochet into the corner space, chain to go over the top of the join, then double crochet into the next corner-space.
STEP 5 Continue on to the corner-space, and work (dc, ch 2, dc) into the corner space.
STEP 6 On your next row, work a double crochet into each chain-space, with a chain between each dc.