Two women died when a wall of mud destroyed a house near the Swiss town of Lugano on November 16.
Over the border, a pensioner and his granddaughter were killed when a mudslide engulfed their home.
The heavy rain is expected to continue across the region, and both countries have issued major flood alerts.
The levels of lakes Lugano and Maggiore are already dangerously high.
Homes and businesses near Italy’s Lake Maggiore have been cut off by rising waters.
The region has had more rain in a few days than it would normally expect in a year.
The Ticino river burst its banks near Vigevano in northern Italy.
Further north, in the Ticino region of Switzerland, rescue workers were searching for survivors on Sunday after mud swept down a hillside and destroyed an apartment building.
Swiss authorities said two women, aged 34 and 38, had died when the mudslide hit the building in the village of Davesco-Soragno.
A third person, a 44-year-old Italian, was rescued from the rubble and taken to hospital.
Relatives were also mourning the deaths of a 70-year old man and his 16-year-old girl in Cerro di Laveno in Italy.
Their house, near Lake Maggiore, was hit by a landslide late on Saturday night.
The weekend’s landslides are the latest of many to have hit northern Italy and southern Switzerland amid incessant rainfall over recent weeks.
High volumes of water gushed down the Ticino river, seen here in the town of Giornico in southern Switzerland, last week.
At least 11 people have been in killed Italy because of extreme weather over the last month.
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