My daughter is around the same age (although in high school so maybe a school year above?)
She does ISTD grade 3 - 2 lessons per week. One of these classes is for everyone and one is an exam class. She also does 1 grade 5 class as, like your situation, there isn't a grade 4 class available. So in total she does around 3 hours ballet a week (plus some classes in other dance styles). She manages the grade 5 fine and enjoys it although you can tell she isn't of exam standard for this grade. However, she is not intending on applying for vocational school or associates so I'm not sure if that really helps answer your question!