People keep throwing this number around but it's at best voluntarily missleading, if not straight up a lie.
4 to 6k gallons is roughly what a meat cow (slaughtered at 18 months of age) will use in its lifetime (which btw if a lot less than the average westerner in the same time span). To stretch it to "4 to 6k gallons per pound of beef" you have to include the rain that watered its pasture, which makes absolutely no sense. Nobody ever tells that a bread loaf requires 500L of water to produce (roughly the quantity of water needed to water 1 bread loaf worth of wheat) because this water falls for free from the sky anyway.
Now you can make intricate arguments about how if there were other types of ecosystems in place of pastures more water could be retained in the soil and would eventually make its way to aquafiers or rivers -which is true- but I'm confident this isn't how people interpret this number. The goal of people who provide this number without any context is to give the impression that eating a pound of beef is equivalent to letting the faucet open for 4k+ gallons worth of water, which it absolutely isn't.