You will distribute the stitches along the four needles and knit with the fifth. Or, in most cases, distribute the stitches among three needles and knit with the fourth. It's weird, most DPNs come in sets of four, however, sometimes you will find them in fives. I've stopped trying to figure out why and just deal with what I have. I am thinking that with the larger needles that they give you five because you will most likely be using a bulkier yarn and need the extra space so your stitches are not constantly falling off the end of a needle.
Your biggest challenge, since this is new for you, will be making sure your stitches dont get twisted the wrong way when you distribute them and start your joining round. Good luck and don't worry if you don't get it right the first time. Keep practicing and you will be doing it like a pro in no time.