[quiddity]

Assuming 3NT shows a balanced hand with a tight range (and I guess it should be pretty precise given the amount of space it takes up) then I think it would be better to play 4NT as rkc. Otherwise it becomes troublesome to handle the strong spade hands which just want to keycard. Meanwhile the loss of a quant bid is pretty minor given that responder has already tightly limited his/her hand.

edit: I see some suggestions that opener should cuebid on the way to rkc but then it becomes troublesome to show the 2-suiter.

[jmcw]

If 3NT agrees a balanced ♠ fit, then, 4NT is RKC.

If 3NT can be with only 2 ♠, then, 4NT is quantitative.

Simple enough and it works, no need to complicate with cue bids in order to "set" trumps.

[TWO4BRIDGE]

 quiddity, on 2011-August-01, 18:22, said:

edit: I see some suggestions that opener should cuebid on the way to rkc but then it becomes troublesome to show the 2-suiter.

I think the 3NT bid has made it impossible to show a 2-suiter.

If the 3NT shows a 3 card fit for the Major, then 4-new suit will always be interpretted as a Ctrl Cue bid;

whereas:
1M - 2m! ( 2/1 GF )
3m - 3M always shows a 2-suiter.

Also, another negative attribute for the 3NT bid is when M = Hearts... now Opener has no room below 4H to cuebid Spades.

[jdeegan]

What about using the otherwise almost idle 3NT response as RKC for spades? It keeps the bidding lower and suggests slam. With all this other 13-15 HCP junk you can start with a forcing NT or temporize at the two level in one of your minor suit 4 baggers.
1. You can afford to ask for key cards and get out at the four level. No serious worries with a small doubleton side suit holding. You will be just with the field.
2. If opener has serious extras, then a 4NT response to 3NT is RKC in spades where five is at worst a safe bailout, six is very, very likely, and seven is in the picture.
3. With moderate extras, opener can suggest slam by venturing to the five level with a cue bid after responder signs off at 4♠.