Do you know of people that feel perfectly refreshed and functional with just 3-4 hours of sleep every night?

Yes, but: This is not the norm. Most people need more sleep than 3-4 hours but some seem to function quite well on very little sleep. The area of sleep is a fascinating one and more knowledge is being gained daily so we don't fully understand why some people do well on a little sleep while others need more. Most people need 6-9 hours regularly, though.