Should you make a 16 year old sit in a safety seat if she happens to be 4 inches under the height limit and is very light?
Yes. The height requirement is not based upon age, but upon the ability of the lap/shoulder belt to cross the body in areas that would cause the least injury in a crash. They should fit with lap belt crossing the hip bones, shoulder harness crossing sternum and clavicle, not the neck. I'm petite, they fit properly in some, but not all cars. Use a booster if her car's seat doesn't fit as described.
No. Technically only if she is under the height requirement to use a seat belt alone. That height is 4'8". If she is taller than that, then seat belt alone is fine.
Yes. The safety recommendation for booster use is based on the height one needs to be for a seat belt to work. In a crash, the lap belt will travel backward , bounce up and crush the midsection of a short kid. In my career i've cared for cases of colon rupture, bladder rupture, intestinal blockage, paraplegia etc. All from kids who relied on a sest belt when they were too short. It's better than nothing.