Puppy potty training issues, help!?

So my sister just sent me her little shih tzu puppy for a few months so that I can help potty train it for her. (she shouldn't have a puppy in the first place if she doesn't have time to potty train it, I know.) The puppy is about 4 and a half months and she has no idea that she is supposed to go to the bathroom outside. I have been taking her outside about every hour in hopes that she will use the bathroom outside so I can positively reinforce her with treats and praise, but she just won't use the bathroom outside, she doesn't even want to go on the gr. I can't start crate training her yet because she just uses the bathroom in her crate. (She lived in a pet store until she was 4 months so she is used to just peeing and pooping where she lives.) I sat outside with her for about an hour this morning and she just doesn't get it, about a minute after bringing her inside she peed on the floor. how do I get her to potty outside?