I'm not sure where to post this so please move if I guessed wrong.

I purchased a ticket from ORD to LHR on UAL/BMI. I need to purchase a ticket on the outside BNA-ORD. It will be on AA. I can take a flight that gives me a layover of either an hour and a half or four hours. I'm inclined to take the 4 hour layover as a precaution. I'm not so concerned about the timing coming back as I will choose a 3 hour layover which should be fine and if not there is a later flight. I will be carryon going so that will not be an issue.

Is there anything I should consider? I would love to take the later flight. The trip is in October so weather should not be a big issue. The ticket on UAL/BMI was the Priceline fare that ran last week so I don't know how things would work if I missed a connection. Thanks for any thoughts as I have never booked 2 tickets this way.