Nineteen people have now died and at least 245 are sick in a deadly outbreak of fungal meningitis linked to tainted drugs.
American health officials from the Centre for Disease Control reported the four new confirmed deaths late Wednesday - two in Tennessee, one in Florida, and one in Virginia.
These latest figures come after Federal investigators from the Food and Drug Administration (FDA) searched the New England Compounding Centre (NEC) for several hours on Tuesday.
FDA spokesman Steven Immergut said the raid was part of the investigation into the meningitis outbreak, which has affected 15 different states.
Nearly all the victims had received steroid injections for back pain supplied by the NECC, which is based in the Boston suburb of Framingham.
Company lawyer Paul Cirel said the raid was unnecessary and that "asking would have produced the same result".
He said: "It is difficult to understand the purpose of this search, since we have been clear that (the NECC) would provide, and has provided, anything requested. We've been clear that warrants weren't needed."
Federal and state investigators have found fungus in more than 50 vials from the company but have not confirmed the source of the contamination.
Nearly 14,000 people nationwide are at risk of infection because they received injections from suspect steroid medications shipped to 76 facilities in 23 states.
The FDA this week widened its investigation into the cause of the fungal meningitis outbreak to other drugs made by the NECC.
Meningitis is an infection of the membranes covering the brain and spinal cord.
Symptoms include headache, fever and nausea. Fungal meningitis is not contagious.