Plant officials said the release was contained by 6am.
Several workers had gone to investigate the leak when they were exposed to the chemical, plant manager Randall Clements said in a statement.
“There are no words to fully express the loss we feel or the concern and sympathy we extend to the families of the employees and their co-workers,” Mr Clements said.
“We are in close touch with them and providing them every measure of support and assistance at this time.”
A fifth worker was taken to hospital where he is being monitored. His condition was not known.
Methyl mercaptan was used at the plant to create crop-protection products such as insecticides and fungicides, according to DuPont.
The cause of the leak was not immediately known.
DuPont said would co-operate with local, state and federal officials investigating the leak.
Jeff Suggs, emergency management coordinator for La Porte, said the chemical release was not toxic for those living nearby, but that it caused a smell that’s similar to rotten eggs.
“It’s a nuisance smell in the area. It’s a smell that’s travelled quite far,” he said.
The La Porte plant has 320 DuPont employees. Four other companies are also tenants at the complex.