It does not matter if you have 4 or 5 dpns. Traditionally you will get 5 if you buy very small size sock needle dpns (it makes it just a bit easier to work the smaller diameter sock), but you can use just 4 too.
Simply divide the total number of stitches by 3 (rather than 4) and cast that many stitches onto 3 of your dpns. Then use the 4th to start knitting.
By all means you can start with the circular needle and use the "magic loop" method. Just make sure your circular has a flexible cord.