For centuries the rosary remained the same and then along came "Saint" JP II and changed it significantly by adding 5 more decades.
Fr. Phillipson of the Fatima Center mentioned recently in a talk on the Holy Rosary that the number "4" is considered by Gnostics to be more important than the number "3." "Fourness" is loved by astrologers and New Agers. The gnostic Carl Jung considered the number "4" to be the most important spiritual symbol of all time.
There are many reasons to believe that Our Lady wants us to pray three sets of mysteries and not four. The Rosary is "Our Lady's Psalter." There are 150 psalms. Three sets of mysteries includes 150 Ave Marias, and when you count the 3 said at the beginning of the Rosary, you get 153, which is the exact number of fish St. Peter caught after Our Lord's Resurrection (St. John 21:11).
The part about the traditional Rosary vs. the new four mystery Rosary starts at 29:20