Fervorous Beastwaker

For as long as you control two or more creatures with one or more +1/+1 counter on them, you may pay {1}{r}{g} rather than pay Fervorous Beastwaker's mana cost.
Whenever a creature with power 4 or greater enters the battlefield under your control, add {g}{r} to your mana pool.
{r}{g}: Target creature you control gets +1/+1 and gains haste and trample until end of turn.