It is not necessary to do that at 4. The 20/40 vision means when a person can see something at 20 feet, but the other one can see the same thing at the same place at 40 feet. That is to say, the eyesight is two times worse than that of the normal people. However, the 20/40 vision is normal to children who are 4 years old because the eye vision is changing. The eye vision can be stable when they growing up to 20.
It depends on you. As we know the perfect vision is 20/20, and 20/40 vision means that her vision is not good, yet not very bad. If she don't feel vision blur or difficult to focus blackboard, you needn't buy prescription eyeglasses to her. But if she feel vision blur in daily life and can't focus objects she pay attention to, you need buy prescription glasses to help her for better vision.
You said your child's eyes have 20/40 degrees after eyes checking, but I suggest you do not let your child wear glasses too early. First of all, 20/40 degrees is not strong prescription, and it can not be a big problem for child to see things. Second, the function of the glasses is to reduce muscle contraction of the eyes to see the same distance things. If she wore glasses, the possibilities which her sight would restore through the eyes exercise. Third, some children do not like to wear glasses, because that would make them be discriminated by their peers.