"We are trying to find out whether they fled by climbing the prison wall or simply walked out of the gates," Jail Superintendent Dev Kishun Singh said.
The escape of Aman Singh, Vishal Srivastava, Arman Ansari and Botal Yadav was discovered during the morning head count, he said.
They were accused in the murder of an under-trial prisoner Kamlesh Yadav in court premises two weeks ago, Singh said.
The four prisoners were arrested from the Danapur Railway Colony in Patna district.
"We have launched an operation to trace their whereabouts and bring them back," he added.