Why the discrepancy in the version numbers?
Once upon a time there was just node.js then some people were unhappy with the speed of releases of node.js so they created io.js.
A few months later io.js converges back with Node.js 0.12.x into a unified Node.js 4.0.
Hence version Node.js 4.0 is born.
4.0 is the new 1.0