It's quite simple actually: 4m is RKC with that minor.  This way you stay low and don't get pushed into slam when partner has a certain ammount of keycards.

Responses are like normal RKC (example 1430):
step 1 = 1 or 4
step 2 = 0 or 3
step 3 = 2 no Queen
step 4 = 2 with Queen

After this, any bid in the minor is to play.  The step just higher than the minor asks for Kings, and you can respond however you like (specific Kings, number of Kings, other).

Example:

1 Diamond - 2 Diamonds
4 Diamonds - 4 Spades (RKC Diamonds - 0/3 keycards)
5 Hearts - 5 Spades (asks kings -  SpadesK in case you play specific)

Asking for the Queen is just as usual: next step over 14/30 responses:

1 Diamond - 2 Diamonds
4 Diamonds - 4 Spades (RKC Diamonds - 0/3 keycards)
4NT - 5 Diamonds (Diamond Queen? - no)

Note: in my examples, bidding up till 4m is very poor (no controls), but that's not the point of the examples.