Why there was never a Winamp 4
Winamp, famously, skipped over version 4 and went straight from Winamp 3 to Winamp 5. The stated excuse was that it was a combination of versions 2 and 3, since version 3 had been poorly received, and that “2 + 3 = 5”.
However the real reason, as I understand it, had to do with Winamp theme files, called “skins”. If you ever needed to specify which version of Winamp a given skin was made for, you might have to say: “This is a Winamp 2 skin!” or, “This is a Winamp 3 skin!”.
You may have to say it aloud to yourself to realize why they didn’t want to have “Winamp 4 skins”.
From the Winamp Wiki:
What happened to Winamp 4? You're not imagining things. Yes, we skipped a version number for the following reasons: a) Winamp 5 combines the best aspects of Winamp 2 and Winamp 3 into one player. Hence Winamp 2 + Winamp 3 = Winamp 5! b) Who the hell wants to see a Winamp 4 Skin :P c) We think that a Fibonacci sequence for versioning might be pretty damn cool. d)We improved so much in Winamp 5 that we figured it warranted skipping a version. ;)