There were no 4.x and 3.x. They just jumped to 5.x. Another stupid action that shows they even don't know anything about versioning.
You know what, iOS for the iPad jumped from 3.2 to 4.2, there was no 4.0 and 4.1. And why? Because 4.0 and 4.1 were only released for other platforms (devices).
Same applies here, Skype 3.x and 4.x had been released on Windows but not for the Mac.