Yes, quarto is a Scrabble word!
...and is worth 15 points in Scrabble, and 16 points in Words with Friends
quarto
Noun
1. The size of a book whose pages are made by folding a sheet of paper twice to form four leaves
What Scrabble words can I make with the letters in quarto?
5 Letter Words
4 Letter Words
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3 Letter Words
2 Letter Words