First, place the correct answer/value in a random position. Store this value so that you have something to compare the user answer with as well as to check against a duplicate when picking random values in the next step. For each of the remaining three positions, chose a random non repeating letter by using the advanced random plugin. If the correct answer is randomly picked, then ignore that result and use the next result.
To use the advanced random plugin, first use the create permutation table action with offset 1 and length 26. This will generate the numbers from 1 to 26 and then randomly shuffle them, each one corresponding to a letter of the alphabet. To retrieve the values, use the AdvancedRandom.Permutation expression, where AdvancedRandom.Permutation(0) would be the first value, AdvancedRandom.Permutation(1) would be the second, and so on.
So for each answer choice that isn't the correct one, put the next permutation value in sequence (since they're already shuffled), only if the value doesn't match the correct choice. If it does, skip that permutation and continue with the next.