The next number in the series is always found by dividing the previous number by the number before it. So you calculate the number at position 3 by dividing the number at position 2 by the number at position 1.
40 / 4 = 10
10 / 40 = 1/4
1/4 / 10 = 1/40
1/40 / 1/4 = 1/10
1/10 / 1/40 = 4