I dont think previous comments are helpful at all, a four year old has no concept of sex thats correct but he may not even realise.He is learning about bodies and that bodies and feelings are linked, or as part of learning about his feelings it does not mean he has been abused. He may be scared that you will be angry because naughty children get time out, and that might be the reason he dosn't want the staff to tell you.he needs to be told its not appropriate and i know he's young to be told about innappropriate touching.. you would have the best idea about if it is possible he could have been abused. but four year olds are exploratory and may get feelings which he is testing.
OK so I am going to be blunt here. This sounds like someone has molested your child to say don't tell mom and dad this is all behaviors he has learned from someone meaning someone that has told him to keep this secret and touching other children is not normal behavior from a four year old they have no idea what sexual feelings are yet or privates unless they have been violated. Get the child help!!!! Also you should not be disciplining him for this he needs to see a therapist!!
There is no substitute for their continuing to place him in time out for such behavior. Also, they should have him play away from the girls in question after he has completed his time out period.