The incident occurred at around 9.30am when a 61-year-old man from Valais was driving his vehicle on the main road from Fiesch to Brig, Valais cantonal police said.
In the village of Lax, the motorist stopped in front of a pedestrian crossing to let three children pass by on their bicycles.
After they crossed the road, the driver proceeded through the crossing and knocked over a fourth child — the four-year-old girl — who was following behind the other children on her bike, police said.
Despite the rapid intervention of emergency medical assistants, the girl, who lived with her family in the Lax area, died on the spot from her injuries.
An investigation into the tragedy is under way.