The fourth stitch seems complicated, but once you've done it a couple times and actually go through the motions yourself, it makes much more sense and doesn't seem nearly as complicated.
Remember how the third stitch she taught was basically just using the right hand color and weaving in the left hand color? The fourth stitch is just the opposite - you're knitting with the left hand color and weaving in the right hand color. It just takes a little bit more maneuvering, but it's still just a knit with one color, weaving the back color in. So you'd use it when you're done several of the left hand color in a row, to prevent a long, unwoven section of yarn on the WS of the piece.
Hope that helps, good luck!!