Joyent were incompetent stewards of the node project, so it was forked into "iojs". 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).

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.

Node.js v4 is the first release from the newly merged projects and they used v4 as iojs had already made it to v3.