How is it possible to spend 45-60 min on the treadmill at 3 mph and only record 4 active minutes? I believe the Fitbit flex is not accurately counting active minutes.
The Flex counts changes in voltages in its internal gyroscope, as an old mechanical pedometer counted swings of a pendulum, to identify movement events. These voltage changes are triggered by a physical change in the gadgets orientation. So if you're stood holding on to a bar or with your arms hanging for the hour there will very few orientation changes for it to count.
If this is the case you either need to change your walking style to one with swinging arm movements, ideally emulating a military a march, so there will be an arm swing for each step. Alternatively look for a heart rate monitor, as an app can track activity from an elevated heart rate, or alternatively go for a walk outside, so the GPS in your phone can track some real movement, and the app can have some real data to work out calories from a real distance and speed.
Also to quote the help.fitbit.com site:
That said, because Flex is specifically designed for your wrist, if you move your body a lot and not your arms (or vice versa), you may get a slight difference in activity than you would see on your clip-based trackers.