My neural network is telling me that it's actually too good at 4 mana.
Don't forget about potential combos like having a bunch of zero cost creatures in your graveyard + anything else that triggers when you play a spell or when a creature enters the battlefield. But even without that, this is a card advantage machine.
What zero-cost creatures did you have in mind? Those aren't incredibly common. And in any case, a combo that requires 3-4 specific cards is not overpowered; one that only requires 2 specific cards often is.
FWIW, it's about the same cost as Enduring Renewal.
Even the synergy with Fume Spitter is pretty great. :)
It wouldn't be particularly broken in draft anyway - which is the only meta where rarity does impact balance to some extent.