If I have 4 actions so 4 tokens.
I have a ‘double token’ action. Do I take second token from my pot of tokens leaving me with two or am I still entitled to three as I have three actions left?

Double Power actions put two tokens on a characters but only use up one of your available actions for the turn.

If I’m understanding your question correctly, you should ideally have 8 tokens, not 4.

The important thing to remember here is that there is a limit on the number of costed (non-free) actions a player can take in a turn. From a rules standpoint, there is no particular limit to the number of action tokens you can give during a turn. Many players do find it helpful count out their action tokens at the start of their turn (since most costed actions both give an action token and use one action from your action total), but this is just a shortcut, not something mandated by the rules.

As you've already mentioned, you can take a double power action and end up receiving two action tokens for a single costed action. This still only takes one action from your action total.

Characters with the Autonomous ability still receive an action token when given a costed action, but their action does not take away an action from your action total.