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.