So if it can't. Can i rewrite the bios of my 4gb gpu card and edit the bios of my 8gb in the polaris?
Yes, you can flash different bios to any AMD card provided it supports that specific model.
Yes, you can edit bios of 8GB card. Just backup your original bios first, then edit it as you see fit and then flash the modified bios.
WARNING: Be aware that bios modding and flashing is a risky task. Also, if you don't know what you're doing, you'll only be putting your card at a risk for brick.