Authorities believe the prisoner managed to cut through steel bars above the ceiling.
A huge ash cloud forced the closure of Bali's Ngurah Rai International Airport on Monday for 24 hours.
Indonesia's Disaster Mitigation Agency set up an exclusion zone and ordered people to "immediately evacuate."
But authorities say they can't predict exactly when the blast will happen.
The U.S. is advising against any visits to the volcano, warning that an eruption could disrupt flights.
The four men had been jailed on drug and fraud charges.
Ash can be pulled into jet engines, affecting the internal operations of the engine and fuel flow.