Hi , welcome to health care magic
The normal range between periods is 21 to 35 day, if periods occur less often then the person is said to have oligomenorrhoea which is just the medical term for infrequent periods.
Oligomenorrhoea can occur in a variety of settings such as : (1) adolescence - your daughter's age is very important here. During adolescence menstrual cycles are usually irregular . In one large medical study 5% of the girls looked at had 90 days between periods. Your daughter , not having a period for 4 months has slightly exceeded this point which is why your doctor would have proceeded with his/her investigations(2) stress
(3) extreme physical activity (4) inadequate nutrition (5) thyroid
disease- low levels of thyroid hormone , however you indicated that her levels were ok (6) high levels of a hormone called prolactin
(7) substanstial changes in sleeping habits (8) as a side effect of medication (9) due to any issue related to the ovaries
If the period continues to be delayed then your doctor may do further blood tests to check for some other hormones that could impact on her menstrual cycle such as prolactin levels, testosterone
( small amount normally present in females), corticosteroid levels.
It may be she is just experiencing extreme irregular periods
due to her age but it is important to do the investigations to make sure nothing else is going on.
I hope this information is helpful to you.I wish you and your daughter the best