According to the Guinness Book of World Records, a woman from Russia, Mrs. Vassilyeva, had 69 children between the years 1725 to 1765. She had 16 pairs of twins, 7 sets of triplets, and 4 sets of quadruplets. Suppose one of the births is randomly selected. Given that Mrs. Vassilyeva gave birth to at least 3 children (triplets), what is the probability that she gave birth to quadruplets?