Another hand on which thinking of bidding is an overbid.
Yes, we can construct hands on which slam is good.
There are all kinds of issues, including whether partner denies the club A, which he would for me, but if 4
♦ is intended to be a source of tricks, maybe he doesn't.
But the main point, imo, is that you are misinterpreting the slammish nature of 4
♦.
He isn't 'slamming'. He is catering to your having a maximum 3N call....a hand just short of an invitational 4N....in which case he wants to show a willingness to go slamming.
After all, opposite a balanced 20-21, a hand such as Ax KJxxx Qxx xxx shouldn't invite slam, yet opposite the right prime values, slam might be great.....Kxx AQx AKJxx Kx slam is wonderful.
Otoh, you don't have certain 5 level safety nor can you be sure that you can stop in 5 even if it is safe....you may end up at the 5-level still guessing about slam. So you have at least 2 ways to lose by bidding on.....down at the 5 level or down in slam, with only one way to win...reaching and making a slam.
While sometimes one has to take a chance on 5-level safety, imo this should only be done when we think both that slam is a really good possibility and that we can, by risking the 5-level, find out if it is or isn't. I don't see how we can think either let alone both.
If you read 4
♦ as a form of 'courtesy' cuebid, in case you have a max for your bidding, then while the semantic difference may seem slight, it will, I hope, suffice to keep you from overbidding.
Of course, if you followed my thinking, you probably missed a decent slam, hence the post
'one of the great markers of the advance of human kindness is the howls you will hear from the Men of God' Johann Hari
2♦=not one suited positive (at least HHxxx)
2N=20-21
3♦=5+♥
3♥=at least Hxx support
3N=5♥, cog
4♦=cue bid