You will have 3 in your hat, and then use the fourth as your working needle.
The easiest way is to just take one needle, and knit until you have 1/3 the stitches on that one, take your next needle, and knit the next 1/3 and then knit the last 1/3 with your 3rd needle. With the 4th needle, you will now knit with that.
clear as mud??