Hi Saibal,
Download link of CDH4 is available in your dashboard.
To download:
- Goto your dezyre dashboard
- Sign-in with your credentials
- From the course page- Select "Big Data and Hadoop Certification" under My Courses.
- Select the Installation option-1.
- Download the installation document. Inside document you will get the download link of the CDH-4.

Hope this helps.
Thanks.