The stars, as much as 300 times as massive as the sun, were discovered in 2010 in the giant star cluster R136 in the nearby galaxy the Large Magellanic Cloud, astronomers said.
Until that discovery, observations of the Milky Way and other galaxies had suggested the upper limit for stars formed in the present day universe was about 150 times the mass of the sun.
The newly discovered four ultramassive stars are a significant exception to this widely accepted limit, astronomers said.
However, computer models showed the massive stars weren't formed at that size but were the result of cosmic collisions between more average sized stars.
"Once these calculations were done, it quickly became clear that the ultramassive stars are no mystery," Sambaran Banerjee of the University of Bonn said.
"With so many massive stars in tight binary pairs, themselves packed closely together, there are frequent random encounters, some of which result in collisions where two stars coalesce into heavier objects.
"The resulting stars can then quite easily end up being as ultramassive as those seen in R136."