Operators HC-One confirmed 11 people have now died with Covid-19 at Redmill Care Home in East Whitburn, West Lothian.
In addition to those who died, 35 residents and 20 staff members currently have the virus.
NHS Lothian is leading an investigation into the outbreak and another at Milford House care home in Edinburgh.
An HC-One spokesman said: “Our thoughts and sympathies are with all families who have lost a loved one from coronavirus and we are doing our utmost to support them during this difficult time.
“Supporting our residents is at the heart of what we do and our colleagues are doing everything they can to care for residents at Redmill in what are hugely difficult circumstances.
“We have a comprehensive coronavirus contingency plan in place and we are working closely with NHS Lothian and Public Health to ensure we are doing all we can to respond to and overcome this outbreak.
“A very significant amount of resource and senior leadership time is being dedicated to this home so that we can help as many residents as possible to return to good health.”
He added that the home is “well supplied” with medical supplies and personal protective equipment, and that the company would “engage with the NHS daily” and is in contact with the next of kin of care home residents.
Scottish Labour MSP for Lothian, Neil Findlay, said: “The news that a further four residents have died at Redmill Care Home is deeply upsetting and the families involved have the condolences of all at Scottish Labour.
“With so many residents testing positive it is clear that action must be taken to keep staff and residents safe at Redmill.
“I am calling on the Care Inspectorate to begin an urgent investigation into the situation at Redmill and for all support necessary to be given to the staff at the home.”
Scotland has recorded a further 15 coronavirus deaths overnight across the whole country, the latest Scottish Government figures show.
The daily statistics saw a large rise in cases and people admitted to hospital with the virus, as well as an increase in the percentage of positive tests for Covid-19.