Joyent were incompetent stewards of the node project, so it was forked into &quot;iojs&quot;. The iojs project adopted the semantic versioning system, where a new major version number indicates backwards-compatible changes (which mainly happen when they update the version of V8).<p>Eventually Joyent recognised they were on the losing side, and so they agreed to merge back together, under a new foundation which was set up with the help of the Linux Foundation.<p>Node.js v4 is the first release from the newly merged projects and they used v4 as iojs had already made it to v3.