How many of the eight tzitzit stands may break (at the point where the stands emerge from the last knot) before the tzitzit are pasul?
Provided the eight strands are divided into two sets of four, it is okay to have four strings tear from the same set.
However, if two or more tear from different sets, one has to be concerned that the two are the “same string” and that the tzitzis are pasul.
See Shulchan Aruch 12 and Mishnah Berurah 12:1. The ruling follows the opinion of the Rosh.