[wvenable]

A similar sort of situation happened with PHP which is why there is no PHP 6 -- it jumped straight from PHP 5 to 7.

[RussianCow]

And same with ECMAScript 3 to 5.

[WorldMaker]

There was an ECMAScript 4, it just wasn't adopted by browsers. (It was semi-adopted for ActionScript 3.)

[girvo]

I thought the relationship went the other way, that AS 3 was going to be standardised as ECMAScript 4? Not that it matters, I’m likely remembering incorrectly!

[WorldMaker]

Standards are a two-way street, and we're probably both right from different points of view. (Especially, because AS 2 also had a number of the features proposed for ES 4, so even if AS 3 was "solely" inspired to be an implementation of the ES 4 standard, ES 4 itself took inspiration from AS 2.)