I got the Auto-Brightness thing Figured out. Work around the bug.
When it does work (like after resetting the settings as suggested by gforce216), there are still issues. Here's what I found:
(1) If I go from dark to bright place when 4s is on, the screen immediately brightens up (GOOD)
(2) If I go from bright to dark place when 4s is on, the screen takes a very long time to darken down (BAD). Screen dimming may take a few minutes! By then, my eyes have already adapted to the the darkness but the screen is still super bright. :-(
AUTO BRIGHTNESS BUG WORK AROUND: If you are going from bright to dark place (#2 above), you need to turn off the 4s and turn it back on in the dark place. Turning off/on resets the brightness level and adjusts it to the lighting of the location you are in.
This will work until apple fixes the bug. Meanwhile, saves you the hassle on fumbling with the brightness level slider.