Yep you've got it right. A knack is a specific situation that you gain +1 die in, and you should consider the setting when defining it. No point being a goblin basher if you are playing in a setting with no goblins.
And yes, if you roll two 6s before 4 you gain 2 Knacks, but it's important to note that Knacks aren't tied to your stats. They affect one specific situation or type (i.e. bashing goblins) but can be used with any Stat on any roll if it makes sense.