There is a probability of 1/4 that the first card drawn is a spade. That leaves 51 cards, including the 12 remaining spades. The chance that the second card drawn is not a spade is 39/51; then this goes to 39/50 for the third card; and for the fourth card it is 39/49. So the total probability we have: 1/4*39/51*39/50*39/49=0.118685 approx.
But we can also have the probability of not drawing a spade as the first card=3/4; the second card is a spade=13/51; the third card not a spade=39/50 (as before); the fourth card not a spade=39/49 (as before). But 1/4*39/51=3/4*13/51, so the probability of drawing just one spade within 4 cards is 4*0.118685=0.4747 or 47.47% approx., because any of the four cards can be the spade.