Neat mechanic. Might actually be a bit UP.
a 2/3 creature for 4
is pretty weak, and the growth ability isn't all that strong, since it will never work as quickly as blaze or growth. (max +1/+0 per turn). Compare it to Forest spirit and Fire Spirit and I think this card comes out behind.
I'd say cut the cost a little, maybe to 3
or even 2
?