As far as I remember this is how it works:
Starting from 4.x, every 2nd major version of Node will be LTS (4.x, 6.x etc). But we should expect a few of the early versions of each of these major versions to not be LTS - hence 4.0.x and 4.1.x didn't count as LTS when they were initially released.
But Node is now using Semantic Versioning [1], which mean that there will not be any breaking changes between each 4.x release. So 4.2.x is backwards compatible with 4.1.x and 4.0.x. And 4.3.x will be backwards compatible to 4.2.x etc.
So in essence the entire 4.x branch will therefore be LTS counting from today. So even if we in 29 months release 4.22.0, this will not extend the 30 months.