How many letters does the top answer for “a set of four similar things considered as a unit” have?

The most common answer is quadruplet (10 letters).

Crossword clue: a set of four similar things considered as a unit

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

quadruplet 10 letters wn-def

a set of four similar things considered as a unit

a set of five similar things considered as a unit 71% similar → quintuple, quintuplet