4. 1 3 6 8 16 18 76 78

This sequence appears to be pairs of numbers with a specific relationship.
The pairs are (1, 3), (6, 8), (16, 18).
The difference within each pair is 2 (3-1=2, 8-6=2, 18-16=2).
The relationship between the first number of each pair is not immediately obvious (1 to 6, 6 to 16).
The last pair is (76, 78), where the difference is also 2.
If we assume the pattern of pairs continues, the missing numbers would be the first number of a new pair and its subsequent number.
However, the progression from 18 to 76 is not clear.
The question is incomplete or requires external knowledge.