how can I retrieve text messages that were deleted on my iphone 4s?

There is no way to retrieve deleted text messages except through your carrier, if it's even possible.

If the text messages were part of your last backup, you can restore your phone from your phone's last backup. Otherwise, they're gone.

If the messages exist in the backup, then they can definitely be recovered. Now there are 3 things you can do retrieve deleted text messages from iPhone:-

1 - Go to Apple Genius bar and see if they can help

2 - Restore from the backup:- This will get you all the messages you want, but will wipe out everything that came into your phone since the backup date

3 - Get a backup recovery software:- I have used iPhone SMS recovery software and would definitely recommend it. There are other options though, like the one Daiya has suggested, but they are just too expensive. It does the exact same thing for much less.