Do you know this key bid? If not, you may have trouble bidding to the excellent slam...
South opened a Game-forcing 2C, and partner replied 2D (I prefer not to make a positive response without a 5-card suit headed by two of the top three honours). The key bid is now for opener to jump. Following a Game-forcing opener, this sets the suit as trumps, even if partner is void. Partner is now expected to cue-bid any ace she holds immediately or, without one, go to game in the trump suit.
Here, North cue-bid the spade ace and, when South cue-bid the club ace, North could also cue-bid the spade king.
Without being able to set the suit, it is almost impossible to start cue-bidding, and Blackwood (of any kind) won't work, as South holds a void and will not know if partner's ace is the crucial spade ace, or the less useful diamond ace.
Finally, should South worry about his fourth round of clubs?
When you have a very long trump suit, the fourth round of a side suit is rarely a problem and 90% of the time, your loser is covered by the jack in partner's hand, length in partner's hand, or can be ruffed. Even if none of these exist, you can run your trump suit and keep your opponents guessing. After all, they don't know you hold it.
This randomly dealt hand is so close to a hand I set on this topic - it's good to see that they really do show up in "real life", and my students were able to bid the hand correctly to the excellent Grand Slam!
On this deal, West led 10c. Declarer won and drew trumps in four rounds, before laying down another top club. West showed out, but East's club jack can now be finessed by leading a club from dummy.