Is continuous focus a 4.0-only feature?
In low light(or in very bright light) scenario, 'Zero shutter lag' feature won't work  because the camera module and the software together will do some heavy work to capture picture really good. Try taking picture in good lighting condition(during daytime) to test Zero shutter lag functionality. 

In some phones, Continuous focus makes use of accelerometer sensor to detect the movement of the phones to refocus the camera. This may not work when there is very slight movement. You can also check this during good lightning conditions