Those four were very great warriors and regarded as Atirathas in the world. The very Regents of the Earth could not kill them in fair fight.
ते हि सर्वे महात्मानश्चत्वारोऽतिरथा भुवि। न शक्या धर्मतो हन्तुं लोकपालैरपि स्वयम्॥