Guess Number 1A2B

This is a game. The user could guess a 4-digital number. The host will set a 4-digital number, and the user have to guess it. And then the host will hint the user using ?A?B.
'A' means the number is the same and the location is also the same.
'B' means the number is the same and the location is not the same.
According to this hint, the user could guess the correct number step by step and one by one.