We will use permutations from the previous section to see what is going on.
There are `P_4^26` ways of arranging any `4` letters chosen from the alphabet (where the order is important):
`P_4^26` `=(26!)/((26-4)!)` `=(26!)/(22!)` `=358800`
But in this question, the order is not important. Any set of `4` letters chosen can be arranged in `4!` ways.
Hence, the number of different sets of `4` letters is
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