Why on earth are we punishing kids, 4 year olds for god's sake, for expresssing affection for their caregivers?!?!?! Totally ludicrous. If the teacher's aide wishes to avoid physical contact with kids, may I suggest she change jobs, effective immediately?
And no, burying his face in her boobs is not odd at all. For him, as you say Kathryn, it is not at all sexual, and it may just be that he is curious about the woman's body, that's all. If his own mother is flat chested, I can well imagine why he would find this woman's body and chest quite comforting. He's still young enough to have subconscious memories of breast feeding I would imagine.
My own daughter did something similar with a caregiver once: I barely scrape up to a 34 A, the caregiver must have been a 38 DDDDDDDDDD! Of course she was curious!
Clare