The alert came as officials warned more than 3,000 villagers who refused to leave a no-go zone around the country's most active volcano that they risk forced evacuation if they did not leave immediately.
"We will personally remove the residents who refuse to evacuate to their designated evacuation centres," Joey Salceda, provincial governor of Albay, said.
He said the military and police will intensify patrols to enforce a round-the-clock ban on villagers moving within an eight-kilometre danger zone around the volcano.
Another 16,000 villagers living beyond the danger zone would also have to be evacuated as a precautionary measure, added Salceda, if the alert level is raised to five.
|Mayon has been spewing lava and clouds of ash for several days [EPA]
Scientists emphasised the growing threat of ash and lava cascading down Mayon's slopes owing to the increased activity from inside the volcano, audible as far as 12 kilometres away.
"We cannot say exactly when the hazardous eruption is likely to occur," Renato Solidum, the chief government volcanologist, said in a Philippine television interview.
"The important thing is the distance from the volcano. That is why it is important that people not be inside the danger zone."
Mayon oozed lava and vented steam for two months when it last erupted in 2006, although no one was killed by the eruption itself.
But three months later, a powerful typhoon dislodged tons of volcanic debris that had collected on Mayon's slopes.
The avalanche of mud and boulders crushed entire villages, leaving more than 1,000 people dead.
The 2,460-metre volcano, which is famed for its near-perfect cone, has erupted 48 times in recorded history.
In 1814, more than 1,200 people were killed as lava from Mayon buried the town of Cagsawa.