The boosters just sit on top of the seats. They are there to make the belts hit the right spot better. I prefer the full back ones as they do give some head/neck protection. I know that when I was a kid we didn't have all of these safety things and we lived. Nonetheless, I guess it is for the greater good. If there were no laws, people would still hold newborns and some would go through the windshields in crashes.

 

4 and 40 are the minimum for booster seats. Most 4 year olds cannot sit properly in a booster that uses the vehicle 3-point lap and shoulder belt. 5-step test.

My kids will probably be in 5-pt harnesses probably until age 6. A 5-point harness seat is not any less convenient than a booster except for moving between vehicles.

There are lots of types of boosters. Most rely on the vehicle's seat belt to hold the child in place. A very popular one is the Graco Nautilus. It converts from a 5-point seat with high top slots for the shoulder part to a backless booster seat.