No, it's not that the bones are fully developed. It's that they're MORE developed. It's not just hips that play a part in this spinal development; it's also the cervical spinal column, the shoulders (ever notice how a 3 year old's shoulders are much rounder than a 4-5 year old's, in general?) and other skeletal developments that make a child much more suited to be able to a) handle the crash forces and b) fit a belt properly.
I think many children CAN be ready at 4, but many are not. It's quite possible that cultural influences come into that. It's also on the parents-- children need to be TAUGHT how to sit properly and I know many American parents don't bother to do that, maybe they do where you live.
The fact is that there is no general rule where you can say "at this age a child is safe in a booster" and it will apply to every single child. We can generalize (which is why I tend to give a range of 4-7, with more typical being 5 1/2 to 6 1/2) but only the parent can really make that decision. My own daughter was DEFINITELY not booster-ready until she was almost 6. I noticed a marked difference in her ability to delay gratification, self-control, and understanding that some things have to be done, even though you don't want to, to keep you safe and acceptance of that fact, among other behavioral traits, around 5 years 9 months. Shortly after she had her first booster ride and did perfectly. I KNEW as a parent when she was ready. Each parent needs to be aware of the factors, that's all, so they can make that decision themselves.