I almost upvoted Phil's choice, but I can't stand shouting at the bridge table
4
♣ is natural: he has 5=4=0=4, and the reason the opps haven't bounced in their 10 card diamond fit is that East has a ton of losers and sees the vulnerability.
Heaven help us if partner thinks (shudder!) that 4
♣ is.....g.g.gerber!*
Phil's point about our having shown 2 diamond stops is very well taken. Partner should not pull without a good hand. So I picture something like AQxxx AJxx void Axxx.
Grand may be possible but is unbiddable, at least with any pretence of getting there knowing what we are doing. I could bid 4
♦, which has to be a big move in clubs, but I am not going to be happy next round whatever he does, since I have misrepresented the location of my hcp.
* a lot of beginners learn that 4
♣ is ace-asking and while no reputable text I have seen would suggest that 4
♣ is gerber other than in specific auctions, a lot of beginners learn, from other beginners, that 4
♣ always asks for aces. Beginners have difficult with long, subtle auctions, and so using 4C as ace asking is seductively simple, even if my experience is that it is usually used on hands where the answer doesn't actually help much. If you are one who thinks that 4C should or could be Ace-asking, I apologize for any perceived slur, but recommend that you start limiting your use of gerber to sequences in which 4C is a jump bid, and partner has just bid either 1 or 2N in a natural fashion.
'one of the great markers of the advance of human kindness is the howls you will hear from the Men of God' Johann Hari