Hi and welcome to the parenting board!
To answer your questions:
There are psychological tests that can highlight problem areas in a child. At the ages of 6 and 4, unless there are problems already surfacing, I don't know that they would reveal much... .lots of developing going on currently for them.
I don't know that there is a way to prohibit the development of this disorder when a child is predisposed to inherit it. That being said, curbing the full blown disorder is possible. Though no guarantees.
If we parent a highly sensitive child in the most beneficial way for that child (ie meeting their needs in a healthy way, modeling healthy coping skills, participating in family therapy) it will go much easier for them in life.
There is a DBT based parenting skills book called "Parenting a Child Who Has Intense Emotions" that can help parents of young children build the skills and relationship that is most beneficial. I encourage you to read about it here
https://bpdfamily.com/message_board/index.php?topic=200554.0 on the Book Reviews Board.
lbj