Obtained from New Year's Bad Girls or 2016 New Year's Login Campaign
Much as is Four's nature, which Enhance effect you get is random.
Decent if you have no other Enhance card available, but the unpredictable effects make it rather risky to use.
Grants a random 0-20% Skill Boost to any card.
Much like Four's Main Skill, how much you exactly get out of the Skill Boost is random each time...it could be good or it could be terrible.
That said, using Four as a sub is effectively a gamble. Not useless, but certainly risky.