Agriculture and industry can use _a lot_ of water. Likely the 4000l figure includes that.

It accounts for the water used in agriculture, industry, power generation, etc. Those uses completely dominate domestic water usage including for aesthetic purposes such as lawns and golf courses (e.g. around 80% of water usage in California is for agriculture and power generation if I remember correctly with only around 5-10% for all domestic usage).

As you will note, the average German consumes 1,000 liters per day by the same metric which is 2.5 baths per day which is obviously unreasonable if we were only considering direct domestic water consumption.

To be fair, the agriculture is being grown to feed people, so it is fair to include the consumed produce in each person’s water footprint. This is further exacerbated by growing animal feed for animals that are consumed which is even more “water inefficient”. The US is a major food exporter, so it is over-represented in these sorts of numbers, but it is a fair approximation to within a factor of maybe 2-3x of the embodied water consumption of the average American.

Maybe my comment wasn't clear. That figure isn't great - we just divided the total amount of freshwater diverted to some kind of use by the population. That even includes letting water flow past hydropower turbines. It's obviously too big.

But not counting the water used to put food on your table is too small. So this establishes some bounds.