is there a law for carseats for 4 year old?

[stayinbed]

if so what is it?

All children under 12 have to use an appropriate restraint/car seat, I would look on halfords website they have quite a lot of info about car seat "groups" and their suitability (I think they do, might be Argos. One of the big stores anyway).

yes i saw that on argos.
what if a booster chair says from 15-36kg, 6-11 years old, and your child was 15kg and is 4?

A booster with a back is much better imo.

Go with weight/height not age if they don't correspond. There is no way for example that when I was eleven I would have been on a booster seat, too heavy and too tall.

15kg is 33lbs which is 2 stone 5lbs.

My son is 3, weighs about that and is in a rodi.

here