It's rather good at 4 mana, though that could be balanced in the metagame by making it rare or mythic.
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.
You cannot balance a card [in constructed] by making it rarer. You just make it so fewer people can run the deck based around it in a given tournament.
It wouldn't be particularly broken in draft anyway - which is the only meta where rarity does impact balance to some extent.
Magic is not a game that considers all combos a bug, and there are any number of ways to get to infinite mana (or infinite many other things) with less than four cards.
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.