A 4-needle join can be used to add a decorative strip to your work. I have used it to add the petals and feet in my Flower Dude pattern.
The 4-needle join is basically knitting together 3 sts from 3 different needles.
Place the needles holding the feet or petal stitches on top of the right side of your work.
Holding these 3 DPNs together in your left hand, insert the fourth DPN into the first stitch of the first DPN, and then into the first stitch on the second DPN, and then the first stitch on the third DPN.
Knit these stitches together. Repeat until all stitches of the foot or petal have been attached to the body.
Here is a video tutorial of the 4-needle join.