Baby teeth can become loose at anytime, 4 is very young but he is going to be 5 shortly isnt he ?? so I wouldnt be surprised if it is perparing itself to fall out, it can takes months from when the tooth first becomes wobbly for it to fall out. The average age for the first tooth to fall out is 6, but as with everything else concerning kids some will do it earlier and others will do it later. so I wouldnt worry about it hun. xx