Gateway

Serena: February 29, 1930 - February 29, 1950
Clarrise: February 29, 1950 - February 29, 1970
Tyala: February 29, 1970 - February 29, 1990
Iris: February 29, 1990 - February 29, 2010

Same birthday. Died at the same age. Most died on the day of another's birth. They are quadruplets sent from below. Descendants of the rulers of the infinite plain, aka the Gateway.

Angels in disguise, for all except one.