Is it common to lose your baby teeth at four years old?

No. Baby teeth start to fall out around age 6. So 4 is too early, but not impossible. Please take her to a pediatric dentist.
Unusual. Although age 4 tooth loss is not unheard of, usual age is 6 for girls, 7 for boys. Lower front tooth is 1st to fall out. When there is one eruption anomaly there are often multiple eruption anomalies. PLEASE take your child to Pediatric Dental Spcialist or Orthodontic Specialist. They have in depth growth & development training that is not part of a Dentist's traditional training.
Yes and No. It is not common for a child to lose their teeth at 4 years old, but that really is dependent upon the child's physiological development versus their chronological age. If the child develops earlier than normal, then you will see early loss of teeth and adult tooth growth. The usual age of first tooth loss is 6 yrs old.