The crew was working on a fire in Caparra near Taree on Wednesday night when the truck rolled down a hill, the NSW Rural Fire Service said.
The quartet was taken to the hospital, one of them with suspected spinal injuries.
Firefighters are hoping to gain the upper hand on several out-of-control bushfires burning across NSW as winds die down and conditions ease.
Nearly 80 fires continue to burn along the Great Dividing Range from the Queensland border to Victoria, with 32 still uncontained.
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