Source: Tip For Duplicate Players by Andrew Robson 
[box]Consider leading ace from ace-king against a notrump contract,
in preference to fourth from the top[/box]
This will allow you a look at dummy, and partner will give you a signal. You can then judge whether to continue the suit, or to switch (and to what).
Exercise: The bidding has gone 1NT – Pass – Pass – Pass. What should the following hands lead?
(a). A. You do not know whether to attack spades, diamonds, clubs, or even hearts. However you will know much more after dummy is tabled; it is therefore important to retain the lead, in order to make the appropriate continuation.
(b). A. With a certain side-entry (A), you can afford to lay down a top heart. This will avoid conceding an unnecessary trick if dummy holds a doubleton queen (or you can judge that declarer does), and partner holds Jxx or better.
(c). A. Who knows where to look for tricks? Lay down A, look at dummy (and partner’s signal), and you are likely to find the correct attack.
(d). J. There is a cost in laying down an ace (receiving only low cards, and potentially setting up declarer’s lower cards in the suit), so if you have a very good alternative you should prefer it. Here your four-card heart sequence is both safe and attacking; you don’t really need to see dummy to know that it represents a sound start for the defence.