Voracious Hatchling enters the battlefield with four -1/-1 counters on it.
Whenever you cast a white spell, remove a -1/-1 counter from Voracious Hatchling.
Whenever you cast a black spell, remove a -1/-1 counter from Voracious Hatchling.
If you cast a spell that’s both of the listed colors, both abilities will trigger. You’ll remove a total of two -1/-1 counters from the Hatchling.
If there are no -1/-1 counters on it when the triggered ability resolves, the ability does nothing. There is no penalty for not being able to remove a counter.