There is hard data for both boostering and rear facing being safer (although I unfortunately have no links on hand, I'm sure someone will come and give you some
).
There is very little or no data suggesting forward facing harnessing passed 4 and 40 is safer, as it is a relatively new development for people to harness longer. No one really knows if a properly used harness for a child is better or worse than a properly used booster, although there are convincing arguments on both sides.
What we do know is that most kids are not mature enough to properly use a booster until sometime between 5 and 7 years old, and that the various misuses that occur with these children (shoulder belt behind back, shoulder belt under arm, leaning out of position, etc) can lead to serious injuries in a crash.
Good luck finding links! I will look later, and if I can find anything I'll post it