The automaker is expected to start its next round of white-collar job cuts Monday, but the carmaker apparently has fewer staff reductions left to make than has been anticipated.
The layoffs are part of GM's effort to cut 8,000 positions from its salaried and contract workforce to boost cost efficiencies and prepare for changes in the industry.
Numerous layoffs will happen at GM's technical center near Detroit.
"We are not confirming timing".
About 2,300 salaried workers opted for voluntary buyouts, while another 1,500 contract staff were let go, said the source, who described the estimate of 4,000 laid off salaried staff as reasonable.
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"These actions are necessary to secure the future of the company, including preserving thousands of jobs in the USA and globally", GM said in a statement Monday. Of those volunteers, more than 700 employees have been offered and accepted transfer opportunities at truck, SUV and crossover-related operations, GM said.
GM also said in November that it planned to close five vehicle plants, four in the US and one in Canada.
The automaker also is working with Canadian universities and government officials to assist some 3,000 workers affected by the closure of an Ontario plant, a GM spokesman said.
But GM Chief Executive Officer Mary Barra wrote last week: "We are more convinced than ever that our strategy is sound and in the long term".