Section III is the report. They usually give you a scenario which you must relate your answer to if it asks as it says in the rubrix "use your knowledge and the infomation provided". It does't ask for you to use your own case studies but you can if you want, but really its just wasting your time.
Section IV is the extended response and if you read the rubrix, it says "uses your own knowledg and relevant business cast study/studies". So you have to use case studies for a band 6. Also this section doesn't have to be in report format, you could write it as an essay, altough using report format will help the markers see you answered all parts of the question.