Solve this logic number sequence puzzle by the correct digit
8080 = 6
1357 = 0
2022 = 1
1999 = 3
6666 = ?
8080 = 6 means 6 circles
2022 = 1 means 1 circle
1999 = 3 means 3 circle
so 6666 = 4 means 4 circles