30yo female with chronic heartburn 4yrs on protonix (pantoprazole). Still have occasional break thru symptoms. Could I have esopageal cancer?

Not likely. Esophageal cancer is a progressive disease and most do not survive very long - let alone 4 years. Chronic gastroesophageal reflux is a risk factor for the development of cancer and it may be appropriate to monitor you for this, but your age, duration of disease and lack of other symptoms argues strongly against cancer as a cause of your symptoms.
Not likely. Esophagus cancer is rare in young people. Moreover heart burn is not a symptom.it is usually food getting stuck and not passing down easily. You need to be sure that you are taking your Protonix (pantoprazole) daily and also avoid some of the foods which increase risk of heart burn like spicy hot foods and fried foods. Any meals with heavy fat content should be minimized.