Kira baked 10 dozen hot chocolate flavored cupcakes, which means she baked a total of 10 x 12 = 120 cupcakes.
If she's putting 4 marshmallows on top of each cupcake, you can find the total number of marshmallows needed by multiplying the number of cupcakes by the number of marshmallows per cupcake:
Total marshmallows = 120 cupcakes * 4 marshmallows per cupcake = 480 marshmallows.
Kira will need 480 marshmallows to top her 10 dozen hot chocolate flavored cupcakes.