Transportation would be an issue too. Assuming you were targeting individuals across the nation, they would likely be flying into the nearest regional airfield. You would likely have to bus 400 people within the same 4 hour window one or twice a week, so you would have to maintain or rent a fleet of 10 buses and drivers.

Unless you were in a very big city, I don't think the taxi/uber infrastructure could handle a surge of 400 long trips within that kind of window.