They are all fine.
This question was easy, a mere trifle.
Triflingly simple, it was.
Your number 4 is fine, but a little strange a thing to say.
He had plenty of money, but rather than buy large things he spent most of it on trifles.
To him, money was a trifling matter.
Perhaps a trifle old, but not archaic, not formal.
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BTW: Truffles" are different. A rare, expensive, edible fungus. Also an informal word for a sweet, usually involving chocolate.
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