Interestingly if your 4 digit key for iPhone has a digit used twice then it is harder to guess the correct order given the smudge marks:

    4 distinct digits = 4! = 24 permutations
    3 distinct digits (one repeated) = 4!/2*3 = 36 permutations

Note: division by 2 is because of the symmetry of swapping the order of the repeated digit, multiplication by 3 is because it is not known which of the 3 digits is repeated.

Note2: Using 2 distinct digits (both used twice or one used 3 times) is not as ideal as that results in: 4c1*2 + 4c2 = 14 permutations