The four new legislators jumping onboard are Sens. Tom Harkin (D-Iowa), Patty Murray (D-Wash.), Mark Begich (D-Alaska) and Lisa Murkowski (R-Alaska).
The bill, which was written by Sen. David Vitter (R-La.) as well as Lautenberg, would give new powers to the Environmental Protection Agency (EPA) and requires it to test high risk and new chemicals.
When it was first released on Wednesday, the legislation received a mix of support and scorn from environmental groups, some that considered the measure a sell-out to the chemical industry.
Other green groups considered the deal a positive step to curb proliferation of unsafe chemicals.
The bill also calls for reducing chemical testing on animals, a feature for which the Humane Society's Director of Regulatory Toxicology Kate Willett said the group was "grateful."
On Thursday, two former officials from the EPA's chemical safety office called the legislation "a significant improvement" over current law and said it should "rebuild public confidence" in chemical safety.
-- This story was updated with new information at 3:15 p.m.