I have to disagree with pinkelephants. When we were first considering things, we decided it was either 4+ entry or wait until they were 11. IMO (and it is only my opinion) 7+ is far more stressful for the child.
At 4, they have absolutely no idea what is going on. They go along and hear a nice story and draw a few pictures etc then Mummy comes back to collect them. If they don't get a place then so what? They never need to know or be bothered by it.
However, at 7, the children are far more aware of what is going on and can often sense their parents' tension. They know it's a test and they're always anxious to pass. They're also much more likely to ask about it afterwards. I have taught in an infant school where a significant propartion of children went on to prep at 7. I saw it first hand and it really put me off.
At 4, mine had no idea the significance. I did not prep and they took it in their stride. At 7, perhaps my confident, bright girls would have been fine but their very shy, sensitive brother would have crumbled.
Oh and many admission tutors who assess at 4 will tell you that they feel quite confident at assessing potential at that age. Although the assessment is very play based, it is carefully thought through.
When I taught Reception, you could pretty well tell who would get L5 in Y6. There would be some whom you knew would struggle and need extra support. Then there would be the vast majority who you needed to wait and see how they would develop. But there would always be 3 or 4 who you knew were going to be high achievers.
It is these kids that the highly selective schools are picking out at 4.