You can select the 400% Kill Score Multiplier in Hard Infiltration Operations within the Invasion event,
and continue to play with the multiplier until reset, even after you have cleared the Invasion Dungeon.
As an additional reward for clearing these dungeons, you will receive Standard ETA Vouchers that give you access to a rotating shop NPC.
However, please note that there is a limit to the amount of additional rewards that can be earned, and you will not receive any rewards earned in excess.