The Israeli military had no immediate comment on Sunday, but Jenin has often been the target of Israeli attacks.
The Palestinian sources said an Israeli undercover unit fired at a vehicle carrying four members of al-Aqsa Martyrs Brigades, a group in Palestinian President Yasir Arafat's Fatah faction, killing the men.
The incident happened a day after a Palestinian was killed by Israeli troops after he shot at three Israeli occupation soldiers in the northern West Bank.
An Israeli military source said the Palestinian was shot dead after he opened fire at the three soldiers, killing one and wounding two others.
Palestinian security sources confirmed that a resistance fighter had been killed by Israeli troops in an olive grove in the Palestinian town of Illar near Tulkarim, but his identity was not immediately clear.
The other two soldiers were said to be only lightly wounded.
According to Aljazeera's correspondent, the body of a Palestinian farmer, shot many times, was found after Israeli troops withdrew from the area.
The Palestinian deaths brought the number of people killed since the beginning of the Palestinian uprising or intifada in September 2000 to 4563, including 3259 Palestinians and 960 Israelis.
Elsewhere on Sunday, Israeli troops demolished two houses belonging to activists of al-Aqsa Martyrs Brigades in the Jenin refugee camp.