Automated Bulls and Cows Player
Pyth - 11 bytes
This will eventually guess the correct answer. On average 2500 tries. Literally just takes random guesses no matter what the input is. Since you can't define rateGuess in pyth just takes (uneeded) input from stdin and outputs to stdout.
Ofql{T4^UT4

O         Pick random
 f        filter
  ql{T4   set length is four (checks for duplicates)
  ^  4    Caretesian product with 4 repeats
   UT     Unary range till 10