It's actually 500%, not 400%.
Can anyone prove otherwise? Because despite packfile information having a 400% value and the skill's information window saying 400%, if you actually look at the damage you are doing the result is always 3 hits that add up to 500% damage.
All I can think of is that maybe some other skill influences ninja damage or nexon is just being nexon.
Also, in case anyone was curious about the title and reforge, the following conclusions were drawn after extensive testing:
r1 Kunai Storm = 500%
Master Kunai Storm = 515%
Master Lv 20 Kunai Storm = 535%
In other words, the reforge and master title are pretty much worthless but the skill itself does more damage than it should be doing.