A bid of four notrump is RKC for the agreed upon suit, or in most auctions for the last bid suit. Instead of just four aces, you also count the king of trump as an ace. (Probably better to think of them as five keycards, rather than five aces) There are two ways to respond to 4NT...
Either 0314 or 1430. It's probably better to stay with the more popular 0314 method so you won't get screwed up when you play with other partners.
What does 0314 mean?
Answering 5
shows zero or three keycards,
hence the 03 part.
Answering 5
is one or four keycards,
which is the 14 part of 0314.
Holding two keycards without the queen of trump, bid 5
but bid 5
to show two keycards with
the queen.
Rarely you can answer 5NT, which would be all five keycards, but it doesn't says anything about the queen.
After the response to 4NT, bidding 5NT asks for kings. Sophisticated partnerships respond with specific kings, but most players just drop back to old fashioned bidding and show how many kings they have. (But in RKC you must suppress the king of trump, as you've already shown it.)
If partner indicates 0314 keycards, you can ask about the queen by bidding 6
. (Unless clubs are the trump suit!)
How does partner deny the queen? By returning to the trump suit.
Bidding anything else shows possession of the queen.
Last point... If you are holding three keycards and respond 5
to partner's inquiry, She doesn't know by
your answer if you have zero or three, right?
Now, if she trys to stop at five of the major, you must clarify your holding by bidding the slam if you have three keycards. Go ahead and pass with zero.
Oops, one more...
At the top of this discussion it says RKC can be for the last bid suit.
Here's an example:
1
P
2 P2
P
4NT P
In this case responder bid of 4NT is agreement on hearts, since a jump to Keycard always agrees on the last bid suit.
Most of the bridge experts in the world today use Roman Keycard, and now you can too, but I think I have a better way... (I have no ego at all, nor shame, for saying this! :)