I have taken the exam twice now and made 4's on the writing section both times. I've never claimed to be a right brained individual and as such, have never really been a good writer. I approached this section as if I was a grader of the exam and what I would look for to make a score of 4.
First, you want to make sure you follow the directions and you don't add any superfluous information to make their life any harder. I made sure I had an intro paragraph with a clear thesis and three main points that I could talk about in entire paragraphs down the road. Then I wrote 3 paragraphs, which were generally short and restated what I said in the thesis statement and had supporting statements. Last, I wrote a conclusion paragraph that restated everything I said before and maybe a suggestion to the issue. If you don't have many grammitical errors this strategy should get you a score of 4.