Good evening! I am now an official knitter!

I have already made one scarf and am started on another sampler scarf!
I am also in the process of making cell phone cases for Christmas and not sure how to do something. The directions say, "Bind off all but last 4 sts. Work these 4 sts in Seed st to make strap 5-1/2". OK - how do I do that? I did the binding off, but not sure how to work those 4 stitches. Should I cut the yarn after the bind-off or not? I haven't but if I try to then work the last 4 sts the yarn from the bind-off comes loose. How is this accomplished, please?
Thanks for helping!