At four, my daughter became afraid of toilets. I kid you not ... toilets. It's a horrible fear to have because you can't avoid them, unless you're living in the wilderness. Hers did stem from an experience, which involved a fluke where some washer slid off of some pipe while she was on the toilet and made a huge grinding noise and made the toilet vibrate. So, I was fortunate enough to know the root of her fear. This lasted, until she was six. It involved her getting off and on the toilet as fast as she could, and we had to flush for her. She flatly refused to do it. Then, one day I was reading a John Rosemond column about a little boy who was afraid of newspaper. John advised the parents of the little boy to not react when he showed the fear. He said when he broke down in tears at the sight of newspaper that they were supposed to say, "There's nothing to be afraid of, if you don't want to be around newspaper, then go in another room. I will be here to keep you safe from anything that might be dangerous, but newspaper is not dangerous." That was not exactly the quote, but it was something along those lines. He said if they removed the newspaper or "protected" him from it, then it only reinforced the fear. After that, we quit acknowledging her fear, and honest-to-Betsy, she let it go too. Toilets and bathtubs are harder to avoid and more necessary than newspaper, but I guess you can still apply that basic principle. Good luck.
Take care,
J.