User Manual Fitbit FB417BKGY Charge 4 Special Edition Activity and Activity Tracker

Charge 3 bands are interchangeable with Charge 4 bands.
They're the same exact size.

Mine does not stay on all the time. Maybe it is a setting that I
have not seen.

Yes. There is a Fitbit community via the app. You can add your
own group of friends for challenges.

No, the average battery life is one week, sometimes longer.
It is in constant contact with your skin and is collecting
information to be transmitted to the app. If you only want to
use it for exercise, then maybe it will. The reason people
use these is to get a more accurate and overall idea of how
far they walk, how many calories they burn, how many
steps they climb. If you just want an app for exercise there
are many for free

I'm not a runner and I don't need the GPS feature so I've
turned that feature off. I've used my Charge 4 for yoga and
my elliptical machine. The Charge 4 does all I need it to do.
There's a learning curve that was frustrating at times, but I
think I've got my Charge 4 figured out now.

From the best I can tell from all the internet research I've
done the only difference in the 2 fitbit is the band or strap
material. Otherwise they are exactly the same. They give
you a fancy band and charge you more. Otherwise the fitbit
4 is identical the the fitbit 4 special edition.