I was allowed the Storm Boomerang (Princes of the Apocalypse) on my Kensei Monk character and decided to make it my Kensei ranged weapon. But I am unsure if the Kensei Shot ability would add 1d4 for each damage type the weapon deals or just one, and if only one, which?
Kensei's Shot. You can use a bonus action on your turn to make your ranged attacks with a kensei weapon more deadly. When you do so, any target you hit with a ranged attack using a kensei weapon takes an extra 1d4 damage of the weapon's type. You retain this benefit until the end of the current turn.
Weapon, range, uncommon
Simple weapon, 1d4 bludgeoning - range 60/120 ft.
This boomerang is a ranged weapon carved from griffon bone and etched with the symbol of elemental air. When thrown, it has a range of 60/120 feet, and any creature that is proficient with the javelin is also proficient with this weapon. On a hit, the boomerang deals 1d4 bludgeoning damage and 3d4 thunder damage, and the target must succeed on a DC 10 Constitution saving throw or be stunned until the end of its next turn. On a miss, the boomerang returns to the thrower's hand.
Once the boomerang deals thunder damage to a target, the weapon loses its ability to deal thunder damage and its ability to stun a target. These properties return after the boomerang spends at least 1 hour inside an elemental air node.