Fill in the blanks 1, 22, 2, 44, 3, 55, (), () Why?

Fill in the blanks 1, 22, 2, 44, 3, 55, (), () Why?

4、66
It's actually two sequences
Sequence 1: 1, 2, 3, 4
Sequence 2: 22, 44, 55, 66