The series has two interleaved sequences: Odd positions: $1,\ 2,\ 3,\ 4,\ ...$ (increasing by $1$) and Even positions: $4,\ 5,\ 6,\ 7,\ ...$ (increasing by $1$). The next term is in an odd position, continuing the sequence $1,\ 2,\ 3,\ 4,\ \mathbf{5}$. Answer: $5$.