[NTG-context] mark ii/iv

In the most recent release notes on contextgarden it is said: "This was the first version that separates between a 'mark ii' and 'mark iv' version." I guess I missed something important, but I am ignorant on the terms "mark ii" and "mark iv" version. Searching contextgarden did not reveal an explanation to me. I would appreciate if someone can point me to some reading stuff. met vriendelijke groet Hans van der Meer