24-hour transit and 144-hour free transit in Shanghai Pudong

Hello!
I am an American tourist transiting through Shanghai Pudong on my way to Hong Kong where I will stay 4 days and then I'm flying back to Shanghai and staying in Shanghai for 4 days before leaving for the USA. Will I need a visa? Or can I use a 24-hour transit on my way to Hong Kong and then a 144-hour free transit for my stay in Shanghai?

Thank you!
Al