If I were opener I would bid 3NT on pretty much any minimum hand without six spades, so the 4♥ bid is either a 6-2-1-4 minimum or some sort of cuebid for spades. Since this ambiguity is present, I would bid 4♦ with slam try in spades if I were opener regardless of my diamond holding. As responder with little slam interest and facing the 4♥ call with no discussion, I would never pass, since 4♠ will always be a reasonable contract and 4♥ might not be one. As opener facing the 4NT call, I would respond to RKC in spades. Even if partner intended it as RKC for hearts, I bet he's just as interested in the ♠K as the ♥K.