@Venky4c2, it is very difficult to easily sum up all the source code changes that have happened within all those individual software products between that many major/minor release cycles, but I can hopefully provide some overall guidance which might steer you in the right direction so you can dig down into each component and read the various changes that might affect your environment.
As a high level overview, we have this document, which details the general process of upgrading between CDH 4 and 5.
If you want to peel back the layers and do extra due diligence (which is recommended in production environments), you can find out exactly what was deprecated and what bugs were fixed with each component between each minor release by reading their release notes, which can be found in our archive directories at http://archive.cloudera.com/
For CDH 4.x release notes, you'd want to look here. For example, here is the Flume Release Notes for CDH 4.7.0. These HTML formatted pages should guide you through the developer-oriented details you are looking for on a component-by-component, release-by-release basis.
Once you get into the CDH 5 versions, you'll need to navigate this directory for the various release notes.