I can't believe the amount of broad brush generalising that is going on in this thread. No, not all children who are just 4 when they start school will necessarily be small, scared, tired, lack confidence or social skills, be behind their older peers etc. Just as not every child born before Christmas will be big and tall and confident and outgoing and bright. Some might fit all or some of the stereotypical criteria, some might fit none of it.
Each child is an individual, despite the time of year they were born, and should be viewed as such.
When my DS started school aged 4yrs 2 weeks he was by no means noticeable as the smallest in the class. He was of average size and I'd say, one of the, if not the most confident, verbally able and outgoing children in the class. He could read and write by the time he started school too, which is nothing remarkable in my view. However, he was not able to dress himself very well in comparison to the others but I put this down to always having done a lot for him and not making him do it for himself. He soon learnt this skill though.
There was a girl in his class who turned 5 in the first week of school. She was huge compared to all the other children, she cried bitterly and clung to her mother every morning for the whole of the first term at least and all this despite being very familiar with the school as she had 2 older sisters there. She was a slow learner and although she didn't, and still doesn't, have any specific or statemented special needs, she remains below average in terms of academic achievement.
There were also many children in his class older than him (well they were all older than him anyway) who had to spend most of the day sitting with the teaching assistant because they lacked the confidence to join in with the others, cried for their mummies or had little "accidents" and needed to come home wearing different trousers/skirts etc. Nothing like this happened to my DS though.
So what do we make of all of this? I say nothing. It just goes to show that all children are individuals who do not need to be pigeon-holed by half-baked research in order to be understood.
It is a fact of life that groups of children will be more or less able than eachother in many ways. They all have different strengths and weaknesses, qualities, skills and abilities. This is affected by a list of variables too numerous to be listed, let alone fully understood.
Listen to and learn from your own child and do whatever you can for them. Education does not begin and end in the classroom.