I thought slow cooker chicken breast would always be dry, but a friend in Phoenix swore by a 4-hour method with a cup of broth. | Easy Slow Cooker Meals - Librear
My buddy in Tucson does the same thing. The real trick is to use bone-in, skin-on breasts. That extra fat keeps it from drying out over the long cook. Four hours on low with plenty of liquid is the sweet spot for me too. Anything longer and you're just making chicken string.