What happened with the version numbers? I could have sworn Node was on v0.10 just a couple of months back?
Joyent and the io.js team reconciled and merged their codebases, maintaining everyone's versions. Since io.js had used versions 1 - 3, the merged version number is 4.
I always found the naming-and-proclaiming of semver in the javascript community silly. Old style C libs have been doing what semver describes for decades, and while the js community talks it up like crazy, the compatibility and stability is terrible (and as a result they find the need for private recursive sub-dependencies, and the resulting thousands of unique libs/copies is impossible to audit or patch).
I believe the io.js community was in the "ship or get off the pot" camp, and found it silly that Node was a critical piece of infrastructure at firms like PayPal while still technically not having had a GM release. By forcing the issue, they got Joyent to commit to major version changes twice a year, which sounds like a reasonable compromise between everything-changing-all-the-time and we-can't-have-new-things-because-legacy.
Or consider gtk+. 2.x was backwards ABI (binary!) compatible for over a decade. Compatibility was broken once in the last decade, for 3.x (for applications. for themes and windowing environments is a different story unfortunately.) So programs written for 2.10 or so still work today with (still continuing) recent releases of 2.x.
After all, when a project goes from 2.0 to 3.0, you expect something big to have happened -- not just some breakage.
Some projects use release names (e.g., Ubuntu and Android), but if every project starts using release names we'll have a lot of (mostly silly) names to deal with on a daily basis. Plus, there's no inherent ordering in names.
Except that Chrome is mainly an application for end users and not a library, so the implications of "breaking changes" are a lot less severe anyway. In the parts where it does provide APIs (e.g. to websites or add-ons), the compatibility policies are a lot stricter than semver would require.
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.
lucio was probably referring to this part, where "incompatible" is indeed correct.