As this riddle was described to me, a 4 year old will solve it in 5-10 minutes, a programmer will solve it in an hour, and you will solve it in.....

9313 = 1

7756 = 1

6855 = 3

9881 = 5

8096 = 5

7111 = 0

2172 = 0

6666 = 4

1111 = 0

3213 = 0

7662 = 2

5531 = 0

2222 = 0

7777 = 0

9999 = 4

8809 = 6

0000 = 4

3333 = 0

5555 = 0

8193 = 3

2581 = ???

Spoiler buffer.

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