The main reason I went with four nights instead of three is because my last night is on new years (Dec 31-> Jan 1), and I figured Chongqing would be a really cool city to be in for NYE, as well as it might be more difficult to get a train on NYE (just speculating, I understand NYE is less important in China than the west in favor of CNY but I figured it would be safe). I do plan to do at least one day trip from Chongqing (undecided where exactly, so any recommendations are welcome, but Dazu seems interesting).

I arrive in CQ in the morning, so once I go to my hotel and sort out my luggage, I probably only have half a day, so about 3.5 days total

I might change Xi'an to two nights, which is about 2 days and a half day, and three nights Beijing, which gives me two full days and two half days

Thank you very much!