My 4 week old daughter has an open sore on her left buttock and is in a lot of pain. How can I treat it?

See your Dr. At that young of an age especially, infections can spread rapidly, so she should be seen by her pediatrician very soon.
M.D. visit. Sometimes a pustule on the skin is caused by staph bacteria. I would advise checking her temperature, keeping the area clean and calling your pediatrician. I would suggest asking about staph and MRSA during the visit.