Was time in the womb identical too? Just because 4 people have the same birthday, doesn't mean they all spent the same number of days in the womb like.
Behaviour of people doesn't usually change, so that people conceived around the same time each year isn't all that unusual. And with induced births now, they coulda deliberately caused labor on the same day for some odd sense of parity or the like.
Problem with probability is given enough chances, even the most absurdly unlikely events will happen. As they say, any event without a zero percent chance of occuring has a 100% chance of occuring given unlimited time.