I think you will be okay as long as they know you need to be back by 4pm. Is the tour leaving an hour earlier or is it 1 hour shorter?
If it is the latter, perhaps find out what is being cut.
If you have covid or the flu or even have a cold, you should be isolating so you don't pass your sickness on to others, it shouldn't matter if you feel bad enough or not.
Numerous reasons, as you will be isolating, you don't want your steward catching it and you want the ship to know so you can get better service to your door. Then there is possible medication if you are over 65,