Feed the Squirrel

When Feed the Squirrel enters the battlefield,
you get an Acorn counter.
Squirrel creatures you control have "{g}, Pay an
Acorn counter: Put a +1/+1 counter on this
creature."
{4}{g}{g}: Proliferate. (You choose any number of permanents and/or players with counters on them,
then give each another counter of a kind already there.)