need to replace all 4 tires when only one is bad

Not unless its a new tire you are buying. I recommend trying to buy a used tire with about the same design tred and tred thickness. will save you a lot of money until you are ready to replace all four.

I don't know about "need" but with AWD I chose to replace my OE tires with (4) General Grabbers and I am not sorry. It was this winter before a long trip east and I wanted max traction after 6 years of wear at only 26K.