How do you turn i phone 4s off when lock button is broke?

You can use assistive touch to power the phone off:

http://support.apple.com/kb/HT5587

Once Assistive Touch is turned on:

Tap the dark circle to open the Assistive Touch menu, then tap "Device" and tap and hold the Lock Screen icon. This will bring up the iPhone's Power-Off slider.

To power the phone back on, plug it into a power source.