(cut for length as it's not really spoilerish)
You're not going to believe this but here goes...
4 - stands for 4 degrees of separation (not 6 or 7 as commonly stated.) Roussau believed that everyone in the world was connected by four different people. So in theory could meet yourself by using four different people connected to you. Example: Your brother's, boss's, neighbor nephew, knows your mirror. Hard to believe, I know but keep reading.
8 - stands for the eight continent, or the only place in the world where you could meet your mirror. He calculated it to be somewhere in the south pacific. Now we know its not actually as large as the other contintents but its used figurtively because everyone in all 7 continents could meet there mirror on the 8th universal continent.
15 - stands for the chances of you acually meeting yourself on this place, as in 15 out of 4,815,162,342. Look at the second set "815." Ring any bells? Interesting huh?
16 - stands for the maximum amount of people that could encounter thier twin all at the same time. (Think about the number of main characters.)
23 - is the number of years apart your twin and yourself are. You are not the same age as your twin. By chance and probablity it takes 23 years exactly for the same genes that made you to be connected again to form another you.
42 - stands for the maximimum number of years your twin and yourself can be alive at the same time. However people do die at diffferent times so thats why everyone isn't dead at 75. For example my twin was born on Sept. 6 2003 b/c i was born on Spet. 6 1980. Now that doesn't mean i will die in the year 2045, I could, but its not set in stone. My twin could die when he is 16, and then the whole thing starts over again.
Roussau and his team (wife, included) went to the 8th continent. You can figure out how they died and why by the numbers and equations. It also fits with every character. You can predict when they will die. There are others on the island who are twins of the survivors, but have been there 23 years longer.
gave me the willies...