Having examined my cambelt and kit on 'Pissy' recently. (that's a car that'd done 27k on those pulleys, but sat for a period of nearly 8 years) my belief is that the 40k refers to the time that the pulleys are 'safe' up to, and the 4 years refers to the time the belts are 'safe' up to. My pulleys were silent, and seemed as good as new to me, no signs of stiffness / seizing whatsoever. The belt on the other hand was showing signs of cracking when flexed.

It's been said that were it not for a new belt being included in the kit, there'd not be any real issue re-using the original belts, so only once every other 'cambelt change'.
Would I gamble that belief on my own car? Well, that's another question. But certainly worth looking at the belt, and give the pulleys a whirl, see if you can hear signs of any grumbling (which I'm certain at only 18k you won't). It's not a long job to whip the indiction and drive belt off, then look at the pulleys, so worth a nosey?