you didn't turn off imessage and your apple ID is still associated with your old 4s.
Here is what you need to do, you need to remove the 4S from your support profile at this website.
https://daw.apple.com/cgi-bin/WebOb...618204c476572a1b5e5fb3518&path&language=US-EN
Log into your support profile, find your 4S and delete it so the association between your Apple ID and 4S are severed.
Thanks a lot for the link--I never really thought about that until I read this thread. I gave my old 3GS to my niece--but I noticed it was still registered under me so I just took it off--and added my 4S.