Carseatblog.com has a comprehensive list of typically developing rear-facing car seats that have a minimum weight capacity of 4 pounds AND have "lower-than-average bottom harness slots (ideally 6″ or lower)".
The American Academy of Pediatrics says this about car seats:
"Rear-facing car safety seats provide the best protection in a frontal crash, because the forces are transferred from the back of the restraint to the infant's back, the strongest part of an infant's body. The restraint also supports the infant's head. Severe tensile forces on the neck in flexion are also prevented by use of rear-facing car safety seats." (Source: Safe Transportation of Preterm and Low Birth Weight Infants at Hospital Discharge)
Please consult your child's licensed physician before making any transportation system options to make sure that it will be safe for your baby and his/her unique needs!