Good my wife didn't read what I've written. Actually my child is 4 years and 4 months old:p
Actually we have always used a special chair to drive the child arround (in Portugal they are required by law also).
Nevertheless, the one that my child uses now (in other car) is just to reduce the height of the shoulder strap. That is wonderful. But how can I install a chair just with the abnominal seat belt. It would simply roll over in case of accident...
My point is that the SL rear seats are small enough to properly accomodate a 4-year-old child, as far as it would have a 3 point seat belt (or shoulder seat belt) with the adequate shoulder height.