When should I start using the booster mode on the Evenflo 4-in-1 Car Seat?

You should consider transitioning to booster mode when your child is mature enough to sit properly without putting belts under their arm or behind their back, is approaching the weight or height limit of their current harness mode (10 - 29.5 kg or 22 - 65 lbs), and meets the booster mode requirements of at least 4 years old and weighing 18 - 54.4 kg (40 - 120 lbs). The booster mode helps position the vehicle lap and shoulder belt correctly for optimal protection. (Pages 53-54)