The operations were carried out four times in a year by trained ninjas.

英訳に間違いが有れば正してください。

作戦は訓練を受けた忍者によって年4回行われた。
The operations were carried out four times in a year by trained ninjas.

ヒント
operationsとcarry outの関係は間違いなし

答え in a yearをにa yearする

The operations were carried out four times a year by trained ninjas.

頻度を表す副詞句にinは使わない

twice a month
three times a week