This is an upgrade 2 of the path 1 for the Ninja Monkey. It costs $300 (Easy), $350 (Medium), $380 (Hard), $420 (Impoppable). Shurikens can pop 4 bloons each and is especially useful if combined with Seeking Shuriken. This allows the Ninja Monkey to deal with crowds twice as well. However, this should not be used as crowd control as it still is not enough to pop bloon rushes.
A 2/x Ninja is the only BTD5 tower that has 4 popping power without boosts. Upgrades such as Full Metal Jacket are not counted, because Full Metal Jacket does not affect four bloons at once, only one bloon being popped by four layers.