Number problem

This problem does solve preschool agers (4 years old) within 5 til 10 minutes, programmers within an hour, an people with higher education ... well, check it yourself!

9313=1
9881=5
2172=0
3213=0
2222=0
8809=6
5555=0
7756=1
8096=5
6666=4
7662=2
7777=0
0000=4
8193=3
6855=3
7111=0
1111=0
5531=0
9999=4
3333=0
2581=?

If you want to know the key to the mystery, so click here.