The official news agency Xinhua said the bomb hit a sugarcane field in Lincang city, killing the four workers and injuring nine others.
It came after China's foreign ministry said earlier this week that a house in Yunnan had been hit by shelling from across the border in Myanmar, where the military are fighting rebel forces.
Beijing has previously warned of a threat to border stability after the dramatic upsurge in ethnic conflict in the remote Kokang region in Myanmar's northeastern Shan state.
The unrest has virtually emptied the main Kokang town of Laukkai, the epicentre of the fighting, with streets in the once-bustling frontier community transformed into a battleground.
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More than 30,000 people have fled from Myanmar into Yunnan province, according to Xinhua.
The agency reported that Chinese Deputy Foreign Minister Liu Zhenmin had summoned Myanmar ambassador Thit Linn Ohn to protest the deaths.
Liu urged Myanmar to "thoroughly investigate" the case and "take immediate and effective measures to prevent recurrence of such incidents," said Xinhua.
Chinese foreign ministry spokesman Hong Lei said on Tuesday that two days previously "some shells fell on Chinese territory, damaging a civilian home".