Diagnosis
The QuadPacer or brushing mode timer is functioning correctly. The 4100 has a 30-second pacer pulse, not an automatic shutoff.
How to Fix It
This is normal operation. The brush provides a short pause (pulse) every 30 seconds to signal you to move to a new quadrant of your mouth. It does not turn off. It will run for the full 2-minute cycle if you continue to hold it against your teeth. The pacer is a feature, not a malfunction.
If That Didn't Work
If the brush actually powers down completely and all lights go out before 2 minutes, the battery is failing. It cannot deliver sufficient current under load. The solution is a replacement handle.
Heads Up
No Sonicare model has an auto-shutoff based on lack of pressure or motion. You must manually press the button to stop it.