As to how common? (1) - (4) are all incorrect grammatically, but I think you already knew that. I have probably not heard it since I was a child. In very casual conversation it's meant to lend an air of finality to the utterance. I would not say it's common at all, at least not among people I know.
Do you happen to know of an American band called "The Shins"? They have a song called "So says I." So I thought at least this expression might be somewhat commonly used. I guess it's not.