Round 4 & the back loop only
New abbreviation alert! Meet blo, which means "back loop only."
Round 4: 18 sc blo (18)
For round 4, that means you need to crochet 18 single crochet stitches through the back loops of the previous round.
First, make sure the right side of your work is facing out.
Then, put your hook under the back loop of the next unworked stitch. The back loop is the side of the horizontal V that's further away from you:
Crochet a single crochet stitch like you normally would, and repeat all the way around the round. You'll know you've done it correctly because you'll have a ridge on the outside of your piece: