In other words, you get a bonus point if you score four tries or more, or if you lose by seven points or fewer.
This is the standard system, used (for example) in the Six Nations tournament (as well as the Premiership). Wikipedia lists three variations to this: as used in the French National League, SANZAAR (South Africa, New Zealand, Australia and Argentina) competitions, and the Australian National Rugby Championship from 2013 to 2016.
© Haydn Thompson 2017