definitely an exceptional behaviour - clock otherwise runs at least as long as specified.
(Unless the backlight is used extremely often, make sure that it is not bright and activated as seldom as possible: press and hold down the bottom right - Settings - System - Backlight, then select the switch-on events and brightness accordingly).
YES, in English and other languages, depending on the configuration.
It does not switch English-French or French-English with a click, or with other languages.
The manual is in English.
I don't think the strap of the Vivoactive 3 fits the 4S. The wristband on the 3 is 20 mm wide, on the 4S only 18 mm. But you may be able to buy a white strap with a silver buckle separately (according to Garmin "available soon": https://www.garminshop.ch/Gesundheit...).
Yes, you can set up the display individually. For example, I put my heart rate, steps and day/date on the watch display so that these 3 data are always immediately visible (next to the time).