As you'll know, Umbraco 5 was retired at the annual Umbraco Codegarden conference during the keynote address.
For people who learned v5 only
This is an overview of what's different in version 4, from a v5 perspective.
The core concepts of document types, templates and razor logic are the same in both versions but there are differences you'll want to know about, along with what is better in each version (in my opinion).
Having taught the Umbraco Level 1 Certification Course in the USA and UK with v2, 3, 4, and 5 my view is that Umbraco 4 is currently the version that best meets real-world needs.
On with the show
Master the similarities and differences between v4 and v5, pick up best practices and hidden gems (and even some umbraco folklore too). Ideal for anyone who hasn't used v4.
Or download it here: When Less is More(mp4)
Or download it here: When Less is More(pdf)