Don't worry, it is only natural to write more on the first two to three questions. It becomes difficult to write at length on the last question. Even if you plan beforehand, you are unable to restrain yourself from writing more on the initial questions. This is because you feel confident and you want to write more and cast a positive impression on the examiner. I cannot say with certainty but I'm more than confident about the fact that the examiner takes this into account. If he is impressed with the quality of initial questions, he is likely to lessen the penalty for the poorly attempted fourth question.

So, take it easy.