Hello aamowry,
In the winter to clean a chickens butt I will usually clean them in my sink or bathtub using warmish water. I then take a blow dryer and dry the feathers, however, if you use this method make sure you blow in the direction of the feathers, otherwise you will damage her plumage. You can also just let her air dry, if you do this I would recommend keeping her somewhere warm and dry, or, I even sometimes put them in the coop to dry, as long as there is a heat lamp/source. You can also dry her with a towel or any other water absorbing clothe.
When a chicken begins sleeping in a nesting box opposed to the roosting area, it is usually a strong indication that your chicken is well over laying age and is getting old. Since your hen is 4 years I would say she most certainly passed her youthful years.