Before the attack Wednesday at a resort in Maguindanao province, police said 33 people had been killed in election violence since Jan. 10. The country will hold national elections in May.
The Philippines is a lively democracy but elections are often marred by violence and fraud. About 130 people were killed during the 2007 elections.
In the latest attack, provincial police chief Alex Lineses said unidentified attackers lobbed a grenade at the late-night meeting involving a mayoral candidate for Maguindanao's Talitay township, Montasir Sabal, and his brother, who is running for vice mayor.
Both survived the blast but two other men - either relatives or supporters of the brothers - were killed, Lineses said. Twelve others in the meeting were wounded.
Military spokesman Maj. Randolph Cabangbang said authorities are looking at politics as the motive.
Last November, 57 people, including more than 30 journalists and their staff, were massacred while riding in a convoy to register a gubernatorial candidate. The attack was suspected to be the work of political rivals.
In nearby Zamboanga Sibugay province, also in the south, gunmen fired Tuesday at a group of local candidates in Tulusan township, wounding one, police Chief Superintendent Angelo Sunglao said.
Two separate bomb attacks against two judges - both of whom escaped unharmed - Tuesday and Wednesday prompted the Supreme Court to ask the Elections Commission to exempt some 2,200 justices nationwide from the mandatory election gun ban.
Supreme Court spokesman Midas Marquez said Wednesday that the same request was already made in January. Commissioner Rene Sarmiento said the election body would meet to reconsider allowing judges to carry weapons for self-protection.
Police said nearly 2,000 people have been arrested across the country for violation of the gun ban.
