What does 'Chain 4 then make single turning chain' in a crochet pattern mean?

In crochet, because stitches have "height" (where in knitting stitch height isn't much of an issue), crocheters add chain stitches to the end of a row to add height, in order to begin the next row.

So, in your instructions, chain 4, then make a single turning chain, would mean to add another chain, turn your work so that you could then begin crocheting the second row. You would then insert your hook into the second chain from the hook and crochet the next stitch (which is going to be a single crochet--sc--due to only using one turning chain)

There is a standard turning chain number, depending on the stitch you are going to use in the next row. [the link shows how many turning chains for each crochet stitch].

For a:

slip stitch = 0 turning chain

single crochet = 1 turning chain

half double crochet = 2 turning chains

double crochet = 3 turning chains

triple crochet = 4 turning chains

and so on.

You see that as the stitch height grows, so do the number of chain stitches in your turning chain.