Mine are trained to go outside during the day, and pee pad during the night. I started them as puppies in this routine but trained them on pee pads first, even outside. Then gradually removed the pads outside and only kept them indoors for the overnight. If they're outdoor trained first, you'll need to give them the scent on the pee pad before they'll use it. Maybe bring the pad outside and let them pee on it, then bring it in.
I'm sure with persistence you'll be able to avoid those 4AM trips outside! And I agree with you butterfly, I think it's a universal yorkie code.
