Ok this is a problem with the device itself I think, go to the Samsung store as this shouldn't be a problem of a new phone

Ok those are just numbers given by the phone based on how much it thinksyou will do on each device. If you use your galaxy watch 4 more then it would show a lesser time as it would drain battery as you are using it

Hello, this normally happens after a new software update wait for a few days it might get better or samsung might send a patch

I am not sure about Jan but definitely next year before May

Most probably in Jan 2021