Children should not be placed into a carseat wearing a snowsuit, or even a heavy coat. The buld of the coat or snowsuit creates too much distance and padding betwen the child's body and the seatbelts, increasing the chance of injury and decreasing the effectiveness of the seatbelts. In the event of a quick stop they will not be properly held in place. The safer solution is to keep a blanket so they can remove their coat and cover up once they are buckled in. I actually have my child remove her coat and quickly cover her with it as a blanket so it's already warm.
Children (and adults too) keep warmer when wearing layers, so dressing your child in several thin layers will be as warm as a large jacket. A fleece sweater is a good option to have them wear inside the carseat, under the blanket or coat.
It is also important to ensure the belts and harnesses are properly adjusted for any added buld from extra winter layers. You should be able to fit two fingers betwen them and the harness, with the back of your fingers against their chest. The crosspiece must still be at armpit height, which is one more reason to avoid a heavy winter coat in the seat, the coat will be pushed up around their face and will affect the placement of the crosspiece, the harness or seatbelt itself, and may even block their mouth or nose from getting as much air.
If your child takes their boots off in the car you may also want some warm socks to keep their feet warm until the car is warmed up.
Please remember that it is important to follow correct carseat instructions and recommendations not matter WHAT age the child is. Even at 4 and 5 they are small and need to be kept safe.