The easy answer is "just do it!" but that won't help you. Have you ever used a round knitting loom? It really is that simple, and it works just like that, but there are a couple of things that may work:
First of all, if you don't mind having a tiny seam at the top, you really can cast on four stitches, knit them, then turn the work and PURL front and back--in other words, work flat for a few rows until you have enough stitches that you feel comfortable putting them on dpns or a circular. You can use the tail of the cast-on to sew those few rows to close the hole.
Second, assuming you want to go without seaming anything if possible, you can treat the very top of the hat like glorified I-cord. Cast on the four stitches, knit them, slide them back to the other end of the double-point, knit front and back, slide them...by the time you get to the third increase or so, you'll have enough to divide. When you're done, you'll need to fluff the top of the hat around to get the stitch tension even.
Once you get established, you really do just keep knitting around. On a circular, it's more obvious. You knit across however many you have on the needle, the first stitch presents itself on your left needle, and you keep going.