Three of the men, all thought to be from London, were killed in August as their group attacked forces loyal to President Bashir al-Assad near Aleppo, according to The Times.
The fourth man was shot two weeks later while trying to ambush an enemy position.
The Times said they were part of a ten-strong group of British jihadists who fought together and joined up with 20 other Britons to fight alongside the Al-Nusra Front, which is allied to Al-Qaeda.
A security source told The Times: "Some of those who went out in the early stages of the conflict have been able to return to Britain, radicalise others, and go back to Syria with others in tow."
The Times named one of the men killed as Mohammed el-Araj, 23, from Ladbroke Grove in west London, whom it said died attempting to ambush pro-Assad forces.
He used the name "Abu Khalid" and photographs of him in Syria show him in paramilitary uniform and brandishing an AK-47 rifle.
In 2010 he was jailed for 18 months by a British court after being arrested during a violent protest outside the Israeli embassy in London.
He had fought in Syria alongside the three other men who were killed by a shell two weeks earlier.
The dead include a man using the name "Abu Hujama al-Britani", The Times reported.
In one photograph he is seen clad in fatigues and clutching an AK-47 assault rifle, with the caption "ISIS shahid from West London", referring to a martyr from the Islamic State of Iraq and the Levant (ISIS), closely allied to Al-Qaeda.
Police arrested two men who had recently returned from Syria in central London last month. They are allegedly linked to a terror plot.
Earlier this month, charges were dropped against two British men, including a doctor, accused of being part of a jihadist group which kidnapped British photographer John Cantlie and his Dutch colleague Jeroen Oerlemans.
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