Well, I can understand your fears, but I think the main priority has to be the child right? If something has happened, you need to get to the bottom of it. I don't think something like this means the worst has necessarily happened, but I don't think it is normal for a 4 year old to engage in oral sex. It's hard, but sometimes we need to put our own fears aside to do what is best for our children. It's up to you what to do, this is just what I would do. Best of luck.