Through no fault on anyone's part, it would be inevitable that your youngest would need special attention in her unique position in the family. Keep in mind that your four year old has never had an opportunity to know herself outside of the context of her group of immediately older siblings. It is difficult enough when two siblings are two years apart and there are only two of them. Sibling rivalry can increase the closer the spacing between children and the larger the number of children. Your daughter is the fifth child in a family where all four sibs are only one year older.
Verbal skills take time to develop.
A child benefits from being listened to, completing sentences slowly at first and being mimicked and encouraged. However, when a child experiences 6 older people who are talking more fluidly to one another, even finding a place to jump in can be difficult. If the child is naturally aggressive, he or she may learn to jump into the conversation, but still feel pressured to speak quickly. It is easy to feel less significant and literally not get a chance to get a word in edgewise when your own verbal skills are not as well developed as the rest of the speakers in the family. Her continual masturbation may be a sign of withdrawal from the playing field altogether.