My 4 year old complains of abdominal pain when she "breathes big" or in other words when she takes a deep breath?

Not enough info: More information about the pain, location, and other factors need to be known before a diagnosis can be made. A phone call to your pediatrician and examination would be needed in this case to elicit the exact cause and potentially provide a treatment plan.

A number of: possibilities exist given the inability of a four year old to accurately isolate and describe symptoms. She could have a lung infection, heartburn, gas, gastritis, costrochondritis or any one of a host of serious conditions . If is resolves within a few hours, is not severe enough to interrupt her play, does not affect her appetite, is not accompanied by fever or rapid breathing, and she does not have tenderness when touching the abdomen then you can wait it out. Otherwise take her in to her do

Pain with breathing: Two things can cause that. Either she has reflux or she has diaphragmatic hernia. Easy way to differentiate old be to give her a trial of antacid and proton pump inhibitor. Is her symptoms improve, she had former otherwise she had latter. For work up on latter you need to see your pediatrician