Hi R.,
I wouldnt bring this up at the next Dr's visit. I would make an appointment with a Dr BECAUSE of this. A 4 year old sweating like this while sleeping is not normal. There is a huge array of disorders that sweating profusely (sp?) is a symptom. I'm not trying to sound like a whacko, but hearing this as an outsider, it seems alarming to me. Especially because you say there is so much sweat you mistook it for an accident. If he were running and playing out in the Texas heat would be one thing, but sleeping inside in the winter? Seek medical advice from a qualified professional.
Good Luck in finding your answer,
Margaret :)