2yr old has not pooped in 4 days, during the first week of potty training. she keeps complaining that she feels something in her butt, but cant poop. at what point should we be concerned? what should we do?

Most often they are having behaviors called stool withholding. It can be helpful to pay attention to not only the frequency of bowel movements but also the consistency and the amount of pain involved. After many years of experience I have found that they do better and have less fear with a low plastic potty with their feet resting on the floor.