My iphone 4S does not turn off. It Automatically restarts.

iPhone User Guide

Press and Hold the Sleep/Wake Button and the Home Button at the Same Time...

Wait for the Apple logo to Appear...

Usually takes about 15 - 20 Seconds... ( But can take Longer...)

Release the Buttons...

http://support.apple.com/kb/ht1430

If no joy... Try a Restore...

1: Connect the device to your computer and open iTunes.

2: If the device appears in iTunes, select and click Restore on the Summary pane.

3: If the device doesn't appear in iTunes, try using the steps in this article to force the device into recovery mode.

Had the same issue. It got resovled when I did following steps: -

1. Connect you iPhone via charging cable to your laptop.

2. Now try to power off the phone (with the cable connected). It will power off completely and will not restart.

3. In the Power off mode remove the cable from the phone.

4. Now Power On the iPhone.

5. Now try to power off again without connecting to the cable. It should stay powerd off and should not restart.

Let me know if this works for you. Seems like a hardware software bug which gets reset by the above method..