How do you jailbreak iPhone 4 firmware 4.2.1?

Im almost positive that iphone cannot be restored to 4.2.1 because its not compatable, this is consittering that iphone 4 has a ios 4.2.6 acustom to factory settings. But if this is possible im sure you could use greenpoison. Or snowbreeze with ipsw file of 4.2.1 to make coustom restore + jailbreak. And it wont mess with baseband