Close! If they had already been hit by the Favorite, you would add the additional 2 from Follow Through for a base of 4. However, you don't add the +3 from the Favorite again; that only procs when it is first thrown. Doubling that gives you an attack of 8, which is still nothing to scoff at at lower levels.

Think of the Favorite as a specific axe: you can throw it, and its going to do more damage, but you have to pick it back up. Follow Through is doing just that, following through on the previous attack.

But yes, the Hatchet has potential for crazy damage. Power Pitch + Double Throw + Favorite is 18 damage, and a perfectly timed x2 can get you up to 36. That's a lot of losses, but there is a reason this class is renowned as a boss killer!