Oops, to be fair I don't think anyone's four year old bears any resemblance to the 18 month old baby they had - ds1 at 18 months, was calm, stolid, placid, silent, jolly and enjoyed sitting flicking through books. Now he is a complete teararse who never shuts up, never stands still, barely makes eye contact - eye suspect because it bores him.
I honestly think it is far more likely he changed because you had another baby than because he had the MMR - ds1 changed when I had ds2, in hindsight, and nobody handled it very well, not even the professionals and certainly not me.
I think it's lovely that you're not pushing the school issue by the way.