Roscoe, your facts are unreliable.
According to the Catholic Encyclopedia,
Boniface VI is listed as a true pope, despite some apparent controversy;
Boniface VII was already known to be an antipope in 1907;
John XVI was already known to be an antipope in 1910;
Benedict X was already known to be an antipope in 1907;
Would you care to revise your assertion, or is "...in 1913 Pius had to get rid of four popes from the past," your final statement?