About Bulls and Cows

In this game, a somewhat advanced version of BAGLES, the idea
is that each player (you and the computer) tries to guess a
4-digit number thought up by the opposing player. A BULL is
scored for each correct digit in the correct position and a
COW for each correct digit but out of position

Are you sure that the Pan Books C64 game is the same from the 101 Book? Because the 101 Book description says 5-digit number. The Pan Books version however features only 4-digits to guess as can be clearly seen in the screenshots.

Check, it's based on the same basic code, but since it doesn't work with 5 digits, I modified the input