Not to worry. This past week the Digital Crown on my S4 broke. I have AppleCare. I called Apple support. They did an express replacement. They will charge your credit card for a new watch then send you the new watch usually overnight. You return the defective watch within 10 days and they release the credit card. If you go to the Apple store, they may want to send it in for repair. In that case you may get a refurbished watch back. Best to call Apple support and get express replacement. You will receive a brand new watch of the same model as yours.
Nah. Just because Apple discontinued the Watch 4 for sale doesn't mean they will replace that to a 5 for warranty replacement. They will replace it like for like. They still do make the Watch 4 for warranty replacement purposes.