Recently purchased an iPhone 4. Someone else email is registered to the phone. Would it be easy to replace the previous owners email with mine?

You mean that during the Setup Assistant, it is prompting you for an Apple ID and Password? Or do you currently have access to the device?

If so, in the Settings > iCloud, is there an Apple ID listed at the top? Is Find My iPhone turned on? If so, then you need to return the device, or get the previous owner to remove the device from their iCloud/FMiP Account.

You need to get the previous owner to remove the device from their Apple ID.

support.apple.com/kb/ts4515

What if you don't know the previous owner

carissashorty wrote:

What if you don't know the previous owner

Then you will not be able to use the iPhone.