Sunni-ruled Bahrain has been shaken by unrest since it quelled a month-long Shiite-led uprising demanding reforms five years ago.
Quoted by the official BNA news agency, prosecutor Ahmed al-Hammadi said the four had attacked the bus during a protest that blocked a highway near the Shiite village of Sanabis in November 2014.
The four intercepted the approaching bus during the protest, set it alight and opened fire on its driver "in an attempt to kill him and those aboard," Hammadi said.
Tiny but strategic Bahrain, which is connected to regional Sunni powerhouse Saudi Arabia by a causeway, lies across the Gulf from Shiite Iran and is home to the US Fifth Fleet.
Despite the crackdown on the 2011 uprising, protesters continue to clash frequently with police in Shiite villages outside the capital Manama.