Astril, the Beastwaker

Astril, the Beastwaker costs {1} less to cast for
each creature you control with one or more
+1/+1 counters on it.
Whenever a creature with power 4 or greater enters the battlefield under your control, add {r}{g} to your mana pool.
{r}{g}: Target creature you control gets +1/+1 and gains haste and trample until end of turn.