Use the title as a free hint. The answer is 9 letters.
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Now notice that there are 8 letters with less than 4 occurances, and 2 with more than 4. Combinations of all the letters with a frequency other than 4 don't yield much progress, but if you look at just the letters with less than 4 occurances, they sum to 9.
Filtering out all other letters, we get
Giving us the solution: