4 Single crochets in each corner
I will show you the result right away. Look at the corners: you see them stick out a bit, and that they have holes in the corners on the inside of the 4 stitches, and the weave gets ‘unbalanced’ a bit also in these corners. You will see that quite clear in the next photo’s, especially at corner A. I don’t like it. So I will not recommend this!
If you do want to do it this way, then this is the way to do it in short. Start with making 2 single crochets in corner A. In this case the first single crochet is the first stitch of side AB. Make 4 single crochets in the corner loop at corners B, C and D, where each 3rd single crochet is the first stitch of the new side. When you’ve come to corner A again, you first make 2 single crochets in corner A, and make the last stitch with your needle.
I hope again this was a clear explanation!