A Terrific Problem

All you have to do is solve the one at the bottom based upon the others. Don't think too much - it's not as difficult as you might believe.

8809 = 6
7111 = 0
2172 = 0
6666 = 4
1111 = 0
3213 = 0
7662 = 2
9312 = 1
0000 = 4
2222 = 0
3333 = 0
5555 = 0
8193 = 3
8096 = 5
7777 = 0
9999 = 4
7756 = 1
6855 = 3
9881 = 5
5531 = 0
    ____    

2581 =   ?  

After a little bit of digging around, this problem is apparently all over the Internets, so I have no idea its source.

I ran across it HERE.