Car Seats

My 4 yr old is less than 40 lbs.. i would like to dispose of the car seat which has belts and straps and replace it with a booster seat with back rest.. what does the tx law require

1. before your first birthday less than 20 pounds rear-facing infant-only car seat or convertible seat used in rear-facing position, secured in the back seat of the car. 2. 20–35 pounds convertible seat used in rear-facing position, secured in the back seat of the car. look for a child safety seat that is recommended for heavier infants. 3. when you are 1, 2, 3, and 420–40 pounds face forward in a convertible seat or a forward-facing-only seat or high-back booster/harness in the back seat of the car. 4. when you are 4, 5, 6, 7, or 8 less than 4’9” (57 inches) tall belt-positioning booster seat (no back) or high-back belt-positioning booster seat in the back seat of the car. 5. when you are 8, 9, 10, 11, or 12 more than 4’9” (57 inches) tall may use an adult seatbelt, but must keep your back against the seat back, your knees bent over the edge of the seat, and your feet flat on the floor in the back seat of the car. 6. when you are more than 12 years old more than 4’9” (57 inches) tall may sit in the front passenger seat of the car with an adult seatbelt.