The Spreading Plague

All non-rat creatures have -4/-4. Whenever a creature enters the battlefield, put a -1/-1 counter on all non-rat creatures. Creatures with one or more -1/-1 counter on them have -2/-2. Creatures with three or more -1/-1 counters have -4/-4.
{x}{b}{b}{b}: Put X -1/-1 counters on any number of creatures.
{9}{b}: This ability cost 1 less to cast for every copy of this card on the battlefield. Create a copy of this card.