I personally suspect it's not that kids are safer or less safe at 4, but that's how the data are collected. When a first responder gets to the crash scene and makes a few quick notes about what's in the car, they may miss the details of whether a kid is harnessed or boostered. 'car seat' or 'not belted' or 'seatbelt' may be all the data gatherers have to go on. So they simply break it down that kids under 4 are harnessed, and kids over 4 are not. There's just not the detailed investigation right there to really know the difference between a harnessed 5 yo with a top tether and properly installed (what are the odds of properly installed and used? 1-5%, of course), or in a backless Cosco at age 3 and 30 (about a 33% chance of being properly used).
And the data from Sweden claiming that harnesses are bad, based on some head-force test we don't have here, are very likely based on very bio-infidelic dummies... their heads rip forward from the torso, while real children's bodies curve around much more with the harness and in the seatbelt (so less of this scary neck load thing than we think there is, and a lot more head excursion in a booster than the booster dummies show).
The main thing about a carseat is that it couples the child to the car so that the car can absorb most of the energy, and you lessen the force of the second and third impacts (there are three impacts... the car, the body into the seatbelt or car interior, and the brain and organs into the skull and bones), a harness, properly used, will do that best (and we're talking normal speeds, not NASCAR where they really do need a HANS device because the harness is attached to the car to completely reduce body motion in a 200mph crash...a harness in a child seat will stretch, the LATCH or belt will stretch, the tether will stretch, it's absorbing a LOT of energy away from the child and not causing huge neck loads).
But, statistically speaking, 4 in a booster is basically as safe (though go to the CDC data and it makes you wonder why little kids of harnessed age suffer car crashes as only the number two cause of death, while it doesn't become the number one cause till 3-4, when most kids move to boosters...?)