Why would my pediatrician want to perform an ultrasound on my 4 year old daugter?

Communication is key. There are many different types of ultrasound examinations and each is done for a different reason. The best thing would be to talk with your doctor and find out exactly what he/she is worried about and what information will be obtained from the ultrasound. It is a wonderful tool, the ultrasound, because it does not expose yur child to any radiation.
Kidneys. If your daughter has had a urinary tract infection in the past your pediatrician may be looking for vesico-ureteral reflux which is reverse flow of urine. The test can look at many structures in the abdomen without exposing your daughter to radiation. It doesn't hurt to ask the reason for the test.
Please be specific. To get the most out of this site please follow some basic rules.Provide background information.In this case it would include what type of ultrasound and what the associated problem was. Then ask a clear question, as you tried, except without the type of US it is not specific enough. We cannot see your uploads and your questions are not linked. You are welcome to start over. This is not a chat room.