At least 10 suspected New People's Army guerrillas opened fire with M16 rifles on the soldiers in the rural outskirts of Davao city late Sunday afternoon, initially killing three soldiers, regional military spokeswoman Maj. Rosa Maria Cristina Manuel said.
The rebels dragged another soldier to a rice field and fatally shot him. Troops found his body on Monday, she said.
"Army soldiers were deployed to rescue him because we thought they would just hold him. But they apparently executed him in the field," Manuel told The Associated Press by telephone.
The soldiers were traveling on motorcycles on the way back to camp from a public market when they were attacked, she said.
The attack was staged in Davao city's Paquibato district in an impoverished region where the Maoist guerrillas have been most active and lethal in recent years.
Communist rebels lobbed a grenade toward an army detachment in Paquibato in September, but the explosive hit a protective net and bounced toward a nearby crowd of villagers watching a circus performance, wounding 48 people. The rebels apologized and compensated the victims.
The four decade Marxist rebellion, one of Asia's longest-running, has left more than 120,000 people dead and held back
The military says the rebels' ranks have thinned to about 4,000 from more than 25,000 in the mid-1980s because of battle setbacks, surrenders and factionalism.