The next number in the series is always found by adding the previous number to the number before it. So you calculate the number at position 3 by adding the number at position 2 to the number at position 1.
4 + 2 = 6
6 + 4 = 10
10 + 6 = 16
16 + 10 = 26
26 + 16 = 42
42 + 26 = 68