Do I really need four?

One front tire on my wife's car is bad. Was told at a tire store that because her Subaru is an AWD that I need to buy four tires. The rears have well over half the tread left. I have no problem getting a pair for the front. What is the truth?
Thanks all,
Mike
It's your wife, what's her life worth? My wife drives a 2016 AWD Ford Edge Sport. I keep the tire pressure where it is suppose to be and rotate her tires every 5000 miles. Her tires wear evenly. When they hit 5-6 years old I replace them.