4.5 year old daughter, white bump on/under tongue. 3-4mm diameter. seems hard. says it is not painful or sensitive. 1cm behind tip of tongue in center

Accident history?: It sounds like a scar from a fall. Has that ever happened ? Time to visit a pediatric or general dentist for an exam.

White lesion: Probably scar tissue from trauma (bit her tongue at some point), but can't tell from just your description. Any white lesion in the mouth of greater than 2 weeks duration MUSt be evaluated. So take her to see your Dentist, or preferably a Pediatric Dental Specialist (Pedodontist) for examination, diagnosis, and appropriate treatment.