===== Answer =====

Next is 1 2 3 1

If you read each line out loud, you will see the pattern. Each line is a description of the previous line.
|1| "One" |
|1 1| "One one" |
|2 1| "Two ones" |
|1 2 1 1| "One two One one" |
|1 2 3 1| "One two, three ones" |