4digits - guess-the-number game, aka Bulls and Cows

4digits is a guess-the-number puzzle game. It's called Bulls and Cows, and in China people simply call it Guess-the-Number. The game's objective is to guess a four-digit number in 8 tries and the shortest time possible. It is similar to Mastermind, but the four digits must be unique. 4digits has both a graphical user interface version and a command-line version.