M.G.
I would be very concerned with how your children literally fit into these seats. There is a reason there are height and weight limits on all seats. I could not imagine why a 4 year old who is admittedly petite needing to be in anything but a 5pt harness. It goes without saying that no the police will not check the weight of your children however if they are not properly restrained that could cause a bigger issue, as in death. Have your children been booster trained and have the ability to sit in a seat belt appropriately 100% of the time? A trip is not a time to test this out if you have not already had this proven. Please do what is safest for your children and look into all of your options before automatically jumping to putting them into a high back booster with a seat belt. There are safer options. Graco Nautilus, many Britax seats, others as well.