If 4-year olds can't sit still for more than a couple of minutes I'm curious to know what people do in real life. What happens at the dinner table/school/kindergarten/day care, etc?
I humbly think we're expecting too little from parents and kids if this can't be solved.
I teach in an early childhood program. I work with 3-5 year olds. I know that Adventuredad has excluded the kids that I work directly with (special needs -- usually autism) from his discussion, but I see the kids and talk to the teachers in the other early childhood classrooms in our site and in our district.
I work long and hard with my students on sitting for meals, for activities, etc. Its definitely one of our big goals because of how directly it impacts their future school success. But, for many of them, they will or should have more support in a vehicle than a booster.
I also see the other preschool aged children in our building. The ones who don't have autism or other special education needs. Some can sit for meals, circle, table, and other activities, but not all. Again, its a priority that teachers are working on. Some children do not sit for meals because there is no table to sit at in the home. We have families with 6 or more children in 2-bedroom apartments or have families who are doubled up with another family. These families might have a couch or something to sit on, but far more important to many of these families than their kids sitting for a meal is that their kids GET a meal or some sort of food. Some families may have a place to sit for meals, but because they are busy working FT, caring for a elderly parent, special needs sibling, etc., meals might be pretty unstructured and on the fly. Many of these same kids also come to school with a lot of stored up energy because they have no safe place to move their bodies or no opportunities to move their bodies. They find ways to move, but not always the safest times, places, or ways to move. For these children, sitting is a skill that comes, sometimes slowly, and often not by age 4.
We have several 4-year-old classrooms in my school building, in addition to the early childhood special classrooms. We have a whole lot of children who are not able to sit still even for short periods of time at the beginning of the school year. Many/most? do learn this skill over the course of the year, but that still puts them closer to or over 5 years old. Many are still unable to do it for longer periods of time. For shorter trips, these kids might do okay in a booster -- as they approach 5, but not when they are 4. For those kids who do not attend a 4-year-old program, there are a large number who start kindergarten who don't yet know how to sit.
Yes, perhaps its that their parents haven't taught them to sit. But, when you need to make sure that your child has food and shelter, sitting still might not be high on your list. We don't know what parents are dealing with at home -- they may be working FT, caring for a special needs sibling or an elderly/ill parent, have some other situation in their lives that make it difficult for them to make sitting still a priority. Some are going to be able to do this, others are going to need more help to teach their children some of these skills that seem so basic.
I think that one thing that we can all agree on is that we need to have both options for this age group that are easy to use and affordable. Some 4-year-olds are going to do just fine in a good booster seat. Others, even those without special needs, are going to need a larger harnessed seat.