Please help - 4yrs old daughter

My 4yrs daughter constantly soils and wee's herself everyday and night and at school, her milk teeth have also started  falling out and has lost 2 , her teeth are very well looked after and there is no decay. She is very skinny and eats  constantly she is 1 of 5 children the others being boys  and is the 2nd youngest.
At school  she is being access for social behaviour as she will not engage with any children or the teacher , apart from one child who she has known from being born , she has been at that school since she was 3 in nursery .
The teachers get no reaction from her when they ask her questions  only on occassions, she has always been a shy child  even at pre-shcool nursery.
She does have  eye contact with some people and is  usually a happy child but often seems in her own world ..but doesnt every 4yrs old? Please  can someone give me some advice .. thankyou