We tested both versions and noticed that the video and audio quality is much better in 4.0. There's also less buffering, and it drops calls and goes silent far less often than version 2.2. Also, it doesn't crash anywhere near as much as it did and your chat history loads more quickly.
Skype has added some useful tools, including a conversation window that lets you see your recent calls and messages in one place, rather than in multiple chat windows. This tool, which has been in the Windows and Mac versions for some time, makes Skype faster to run and easier to use.
This Linux version was released in the same week Skype added 'contact pinning', which lets you pin your favorite people to the top of your list, to its Windows and Mac versions (but not Linux). The bad news is that Skype now shows adverts during calls made in free accounts on every version.