Question about chicken and slow cooking

I’m in love with my slow cooker it makes everything so much easier as I get to set it up and come home to a nice meal. I mainly use it for chicken breasts, the question I have is when I’m searching for recipes online I’ll come across a bunch that say “cook on low for 3-4 hours” and “cook on low for 5-6 hours”, will breasts cook through in 3-4 hours? I thought it would Atleast need 5-6 bc breasts are pretty thick, if it says 3-4 should I pound the breasts to make them a bit thinner?