If you don’t now play some form of Keycard Blackwood, we strongly recommend that you adopt it.
Our experience is that “Kickback” (one step above four of our suit asks for keycards) is superior to Blackwood (four notrump asks for keycards regardless of our suit). But we don’t recommend Kickback except to partnerships that are willing to do a lot of practice.
For relatively advanced pairs, our experience is that opportunities for “the Queen Ask” occur with enough frequency that it’s worth your time to learn and use this method.
Partnerships that are willing to work on their slam bidding will find much more information on conventions that are useful for accurate slam bidding, in Real World Bridge 7: The Slam Zone.