long story short? There was a fork (called iojs) that moved to using semver and it used v1.0-3.0. It was then merged back into Node (with a ton of great changes

long story short? There was a fork (called iojs) that moved to using semver and it used v1.0-3.0. It was then merged back into Node (with a ton of great changes to the leadership and development of Node) and the extra changes bumped it to 4.0. But yeah, it's semver so it's more "we've had 3 cycles of major breaking changes" than "it's a completely new product".