First, to review: Step 1 of the Snowflake Method was to capture the essence of your whole story into one 15-word (or less) sentence. Step 2 was to take this sentence and expand it into a (roughly) 5-6 sentence paragraph, highlighting the 3 (or so) BIG issues in your story. Then, in Step 3, you were tasked with fleshing out your main characters a little bit: name, story summary, goals, motivation, etc.; basically a single page summary of your main characters.
At these stages of the Snowflake Method of writing, you will be alternating between story and characters, and growing each of them together. That's not to say that you won't alter things as you go along, because you will, but this is a more structured way of approaching things. It's your road map, if you will, but you can take a variety of routes to get to the end: a finished first draft of your novel.
So, here at Step 4 of the Snowflake Method, you will be switching gears and going back to your story. Dig out what you had from Step 2. From each of the sentences that you have from this step, you will now be expanding them to form a paragraph. At the end, you will have at least a page, probably more, that details out your story. You'll start to add particular settings that you may want, a little background on your characters perhaps, or other cool things that spring from your creative mind. Basically, you are writing out a solid synopsis of your novel that most people would be able to understand and (hopefully) decided if they wanted to read the whole novel or not.
Do things have to be perfect? No! The whole point of the Snowflake Method is to help you get closer and closer to the ultimate goal of a finished first draft of your novel. If there aren't things that you realize you want to change, tweak, or discard, either you aren't doing it right, or you are much, much better at this than I am. The goal isn't perfection, the goal is expansion - and fun! This whole process should be fun. You should be enjoying getting these details down on the page, working out how your characters interact, and putting them in new places and times. Always, always, enjoy the process. If you are feeling like things have become too much like "a job", then take a break for a few days (or longer). It will always be waiting for you when you return.
So, at the end of this step, you'll have at least a page full of details about your story, and I'm guessing that you'll have new threads to pull at in your brain as well. Take time, mull things over, rewrite your sentences, etc. This, too, will be the basis for a later step and you'll be helping yourself along by getting things as much to your liking as possible.
Okay, final reveal time again. I'll give you an example of one of my sentences from my Step 2, and then I'll put below it my paragraph of that sentence from my Step 4. Enjoy!
Step 2 sentence:
Out camping and geocaching in the deep Wisconsin woods, two best friends unwittingly get drawn into a voyeur's murderous game.
Step 4 paragraph:
Two best friends...plan for a weekend in the Wisconsin northwoods, camping and geocaching. They drive deep into a national forest in their Jeeps and make camp at a rarely-used site. Excited to find a new geocache before anyone else does, they set out in the late Friday afternoon sunlight to locate the geocache. Upon finding the geocache and opening the weather-proof box, they discover a photo of themselves taken just a couple of hours ago as they were setting up camp. On the back of the photo is a set of coordinates.