Can a 4yo be depressed?

[ridingsixwhitehorses]

My wonderful, usually happy 4yo son has been sad this week - he says he is sad and doesn't know why, has started saying he isn't good at anything (he is middle child and his older sibling is always fastest etc but we work v hard at encouraging him and doing things alone with him so he is the only child there and the best etc). Today he burst into tears and said 'I don't want to be me any more'.

I am very sad for him. But beyond that should I be worried and what should I do?

H has always been the most physical and cuddliest of the three children he often comes into bed at night with me and likes to sleep his nose in my armpit.