Great tit clutch size.

This is the first year I have enjoyed the experience of a camera in my garden nestbox. A Great tit is nesting in it, and I was delighted to see it had laid 4 eggs by Saturday. But since then she has started to incubate them, rarely leaving the nest, her mate is bringing her food. This seems a very small clutch, I thought they laid far more. What is your experience of this?