Dr. Greene’s Answer:
Getting a rash under the neck is very common at 4 months because the skin of the neck’s folds rubs against itself. The heat, friction, and moisture combine to irritate the skin and interrupt the normal outside, protective layer. This rash will often stay there until kids are sitting up most of the day, no matter how it is treated.
Sometimes yeast can get into this irritated skin. When that happens, a yeast cream should reliably get rid of the infection within two weeks, but the rash will usually stay. If the rash is spreading to other spots of high friction or moisture, that would be expected, but if the rash is spreading to normal, exposed skin that doesn’t get this kind of stress, it should be checked again.
As for cleansing the area, soap can be irritating. I prefer just using warm water and a very soft washcloth, and then drying it very carefully. Air to the area is very healing, and sometimes a very thin layer of something like A&D ointment can protect the skin.