Yes, in Alan's scenario I was making the (unwarrented) assumption that the four letters and the four digits were all different.
The number of four letter combinations together with four digit combinations is 26^4*10^4, and if it's just the case of guessing what the letters were and what the numbers were, without any consideration of the order, the probability of a correct guess would be 1/(26^4*10^4).
If all of the letters and numbers in each combination are different, then each can be arranged in 8! different orders, giving rise to 26^4 times 10^4 times 8! different permutations, and the probability of someone guessing the correct one will be the reciprocal of this.