Answer: Approximately 0.3% (see details below)
Explanation:
I am assuming that the order of the digits matters, for example, 4102 is considered a different code from 2014.
Since a digit can't be repeated this becomes the number of permutations of 4 elements drawn from a set of 6 (digits 0 through 5) without replacement. The number of possibilities is 6 (first digit) times 5 (second digit guess) times 4 and so on until 4 digits have been selected:
[tex]P=\frac{1}{6\cdot5\cdot4\cdot3}=\frac{1}{360}\approx 0.3\%[/tex]