Instead of bidding 4nt to request key cards (see RKC0314 for key card combinations),
responder bids the cheapest suit above the agreed suit. This allows the partnership always to be able to stop short of
slam regardless of partner's response (for example, if the trump suit were clubs, using 4nt to ask for key cards is impractical, because any response other than 5C would force the partnership to slam). Thus,
4D is rkc for clubs
4H is rkc for diamonds
4S is rkc for hearts
4nt is rkc for spades.