The gorillas are suspected to have been killed by a lightning strike on February 3.
Three adult females and a male infant were found in the east of Mgahinga National Park with large wounds on their bodies indicating electrocution, the Greater Virunga Transboundary Collaboration (GVTC) said in a statement.
A spokesperson for the group called the deaths a “big loss for the species”.
Only around 1,000 mountain gorillas remain in the wild, according to the World Wildlife Foundation, which lists the species as “endangered.”
The four that died were part of a 17-member group, known by conservationists as the Hirwa family.
The Hirwas had crossed the border from Rwanda into Uganda in August last year and had been living in the Mgahinga National Park, which straddles the borders of Uganda, Rwanda and DR Congo.
GVTC executive secretary Andrew Seguya told the BBC: “This was extremely sad.
“The potential of the three females for their contribution to the population was immense.”
He added that the 13 surviving members of the Hirwa family have been found and are feeding well.
Samples from the post-mortem are currently being tested and confirmation of the cause of death is expected within the next three weeks, GVTC confirmed.