I would say (a) at the point where the initiative card says the wizard starts his "fourth" action, because of two reasons:
i) To be on Hold is an action.
A hero may choose to wait and see what happens by taking a Hold
action. He may then go later in the round if he chooses. A Held action
lasts until it’s used. [SWD, p65]
What I read here is that the wizard has already acted for a fourth round, and therefore Quickness has ended (unless it is maintained). A Hold action is an action like any other, that takes place on its initiative card like any other; the only difference is that it allows the character to interrupt other characters that act later (even in future rounds).
ii) The opposite interpretation can result in absurd situations.
If a character has a Held card when a new round starts, he’s not dealt
in. [SWD, p65]
If a power lasted until "the wizard acts", that would mean that a power can last forever as long as the wizard was on Hold. I do not think this makes any sense.
Update: This was briefly mentioned in another recent question about the duration of powers.
Update: thanks to Toast, we can see that Clint said the same thing in the official forums some time ago:
The power lasts until their action comes up. Choosing to perform a Hold action and wait till later to do something
doesn't alter the fact that their action came up to make that choice.