B.W.
Chicco Keyfit is a narrow infant seat. PLEASE MAKE SURE YOUR 4yr old is in a HIGH BACK BOOSTER SEAT. A no back provides no side impact protection and the little body of a 4yr old needs taht support, along with the belt guides, to properly secure him in a crash. A no back booster is for older kids, like 6/8 and up. I know what the box says, but the box is bare minimums, he needs more than bare minimums.
If your middle child needs a new car seat, the Sunshine Kids radian is the narrowest seat on the market. It will rear face to 40lbs (the XTSL will rear face to 45lbs) and foward face to 65lbs (or 80 for the XTSL). Its about $250, but it easily fits 3 across, fits kids int eh harnss to age 7 or so, and is a great seat.
Kids need to remain rear facing as long as possible. The AAP recommends kids remain rear facing to 2 and 30lbs, or the max limits of their convertible car seat. Kids then need to remain in a 5pt harness as long as possible,but the BARE MINIMUM for a booster seat is 4yrs AND 40lbs. Kids under 40lbs are 75% more likely to suffer abdominal and head injuries in even a minor collision, and to submarine under the lap belt. Kids need to remain in a booster seat until they are 8yrs old and 4'9".
The harness should be at or below the shoulders for rear facing, and rear facing is outgrown when the child has less than 1" of shell above the head or the rear facing weight limit is reached.
the harness should be at or above the shoulders for forward facing. A forward facing car seat is outgrown when the ears are level with seat back, the shoulders go over the top harenss slot, or the child hits the forward facing weight limits.