node.js got forked into io.js. io.js made a lot of improvements and changes, while trying to reconcile the fork and the reasons why it occurred. those have been reconciled.
io.js had many contributors and changes, starting at 1.0, they are now on 3.0. 4.0 is the full reconciliation with node.js, as part of the linux foundation.
It's also useful to note that io.js moved fasted in version numbering because they switched to a more semver versioning pattern, whereas Node had been using something a lot more awkward and rather more confusing. It looks like a benefit to the 4.0 jump is that Node will be using semver moving forward.
