The hikers were walking the GR 20 trail - a famous 180-kilometre stretch on the French island of Corsica - when the landslide struck on Wednesday afternoon.
Three people were announced dead within hours. Another seven people were injured, one seriously.
Emergency crews found a fourth body on Thursday at the bottom of the notorious Circle of Solitude section of the trail, where hikers have to descend a steep rock face assisted only by chains and cables.
A 63-year-old Spanish hiker was knocked to the ground by lightning and suffered shock and several cuts and bruises.
"That's the end of the holiday. It's awful," said a young IT worker from Bordeaux, who gave his name as Michel.
"We're stopping here and we're going back to Bastia," the island's capital.
The landslide struck high up on the mountain slopes, 1,500 metres above sea level at what is considered to be one of the toughest parts of the trail that stretches from the south of the island to the north.
(One of the five helicopters sent to the scene. Photo: AFP)
Five helicopters and a team of 50 rescuers and police were sent out after the emergency call came at 3pm.
The area has been hit by violent storms recently, and authorities have shut down the trail until conditions improve.
The incident has been labelled by the French media as the worst to hit the trail, which attracts hikers from around the world and usually takes two weeks to complete.