Max capacity

All batteries lose health over time. Most people will see the health decrease an average of 1-2% each month but it really depends on your usage. Anything above 80% is considered a healthy battery so there's nothing that needs to be done.

Where did you buy this iPhone from exactly?

Apple has stopped selling brand new iPhone X's in 2018 when it released the Xs and Xs Max.

If you bought it from a reseller, then its likely been sitting in a warehouse for several months to a year in which time a depletion of 4% battery health is not out of the question.

3 weeks since what launch? The iPhone X was launched in 2017. Apple stopped making them in 2018. There are no newly manufacturerd iPhone X in stores at this point, so the batteries would be several months to over a a year old at this point.

As such, their batteries would have been in use for the amount of time they have been sitting in a warehouse, and their health can decrease in that time.

Yes that seems high but Apple won’t do anything unless it falls below 80% while under warranty. Don’t be too concerned with it.

Edit: I missed the other responses here. As Phil0124 said, while sitting in a warehouse it could lose health so 4% may not be high. But as I already stated, don’t worry too much about it. Check at the beginning and end of each month and see what your average is.