Most people play that a bid of 4NT asks partner for aces - some form of Blackwood. We shall not concern ourselves with which variety you may wish to play, simply referring to any form as Blackwood. Instead we shall be addressing the question as to when 4NT is Blackwood, and if it is not, what it does mean.
Those who play 4NT as Blackwood in all situations, and have no wish to do otherwise, stop reading now! Part 1 goes through the fairly standard, commonly known sequences which are not Blackwood. In Part 2 we shall aim for those areas only common amongst the more advanced player.
1. Raising no trumps is always invitational. Thus 2NT-4NT simply asks partner whether he is minimum or maximum for his 2NT bid. He passes with a minimum, and bids 6NT with a maximum. This usually extends to sequences such as 1NT-2, 2H-4NT, showing an invitational hand with four spades, and, if you play transfers, 1NT-2, 2-4NT - an invitational hand with five hearts. (The idea is that knowing how many aces partner has is irrelevant for 6NT - you need 33 points generally speaking, and with that number you won't have two aces missing anyway.)
When partner opens 2NT and later bids 4NT this is normally natural, denying a fit. Thus 2NT-4, 4NT, where 4 showed a slam try with diamonds, and 4NT just denied much interest. Playing transfers, 2NT-3, 3-4, 4NT denies much interest in either suit.
2. When partner opens four of a minor, 4NT is natural, suggesting this may be the last making game contract. This is a surprisingly useful bid. Similarly, when partner overcalls four of a minor, 4NT is natural. However, over four of a major, 4NT should be Blackwood, since you are far less likely to want to correct the contract in this case as you are already in game.
3. When opponents open four of a major, 4NT is showing two places to play, that is, at least 5-5 in two suits (with three suits, try doubling them). If they open 4 it is the minors, but over 4 it can be any of the other two suits. If partner doubles their 4 or 4 opener, then again 4NT is showing two places to play, although they will not necessarily be 5-5 this time.
If opponents preempt with only 3 or 3, then 4NT is showing the minors.
4. We start by considering fairly normal uncontested auctions. There is a good case for playing 1-1, 2-4NT as invitational (there are surely plenty of ways to raise a suit and then use Blackwood later?!), but most people don't, so we won't advocate it here. However, the sequence 1-1, 2-3, 3-4NT definitely should be natural and invitational. If you want to Blackwood, start off by cueing 4, and then use your favourite convention. However, when are you ever going to get a chance to show your misfitting 18 count? Partner may still have a minimum opening bid, or may be much better, and the only way to bid these hands sensibly is to use 4NT at this point. Even for those fans of the Baron 2NT, you would presumably start with 1 with five spades, so the same applies.
I would think most of the top players play the above sequence as suggested. More controversial would be the rule that whenever there is an unambiguous cue available below four of partner's suit, then 4NT is natural. If you want to play this, you'd better have a good discussion with partner!
5. 4NT when a minor suit is agreed. Many people feel that when a minor suit is agreed there is not enough room for partner to respond to Blackwood, and still be able to stop short of slam. For this reason, many people play 4NT as discouraging, with five of the minor being a slam try (or vice versa). It also gives you a valuable help at pairs where you can try for slam and still end up in no trumps if it does not seem to be there. Not as easy as it sounds, but quite sensible.
6. Competitive auctions: 1-(4)-4NT. Some people would play this as Blackwood, and it certainly could be useful in that way on certain hands. Most experts would probably tell you that this sequence shows the minors (the all important principle when being preempted is to find the right fit rather than the right level). The more thoughtful expert might add that you can also use it with a slam try in hearts. You wait for partner to pick a minor, and then convert to hearts, and he'll get the message. Realistically, these auctions need a lot of discussion, and I confess that I wouldn't know the meanings of these with many of my partners.
Now consider 1-(4)-P-P, 4NT. Is partner suddenly launching Blackwood?! Definitely not! He is showing at least six diamonds with either four hearts or four clubs on the side. A 5 bid instead would have shown at least 5-5.
There are of course many other sequences we have not covered, but these are the main types of auction. Most people should be thinking of playing those things in Part 1, partly because they are essential to accurate bidding, but also because they are fairly standard. Part 2 is reserved more for those who seriously want to devote some time to bidding, though I've restricted myself to the genuinely useful sequences.