High Level Shenanigans

Playing IMPS at the local club game, you pick up K75 KQ K82 A9842 with all vulnerable. Partner open 1S, you bid 2C GF, and partner now jumps to 4D, supposedly a splinter in support of Clubs, which RHO doubles.

Since you do note really have any first round controls, even though your hand is awfully good, you pass to see what partner will do here. Partner bids 4H and RHO passes, what now?

At the table, the person holding this hand bid 4N, Key Card in Clubs, over which partner bid 7C (a lot) and RHO now wandered in with 7D. This was doubled, although pass seems like a very workable alternative with all the stuff in the majors, and when dummy put down xxxx xx Qxxx xxx, declarer went for 1400 with his x xxxxx AJxxxx x hand to pick up 2 Imps when they played in 6S at the other table.

The discussion after centered around how to get to 7S in an intelligent manner on this auction once the opps decide to save in 7D. 7C is obviously the first choice of where to play the grand, but you were not allowed to play there. How do you find out partner has the very good Spades required to play in 7S, with the probably bad split.

Giving it to a local expert, he said he would have bid 4S over 4H. This almost has to be forcing, since you passed the double of 4D and then bid it over another cue bid. It turns out partner also would have bid 7C over that bid, but now it is a lot easier for you to pass over 7D and a LOT easier for partner to bid 7S.
Partners hand was AQJT4 AJ97 void KQJT, and with Spades no worse than 4-1, both black grands will make.