Let's analyze the pattern in the sequence:
- From 6 to 12, the difference is 6.
- From 12 to 20, the difference is 8.
- From 20 to 30, the difference is 10.
- From 30 to 42, the difference is 1(2)
The differences between consecutive numbers are increasing by 2 each time: 6, 8, 10, 1(2)
Following the same pattern, the next difference should be 14:
\[
42 + 14 = 56
\]
Thus, the missing number is \( \boxed{56} \).
Therefore, the correct answer is \( \boxed{4} \).