I think the final Cylon is going to be fundamentally different from the toasters, the skinjobs, the Hybrids, and the Four. In some way, it must tie together all
of those types. If the final Cylon is not "special" in some way, it would be quite a letdown. I look at it as being the last piece in a puzzle. If it doesn't seem to "complete" the picture, it doesn't work.
As for the Four, I've been going under the assumption they were born like everybody else, but inherited some kind of genetic "program" that imbues their Cylon-ness. Imagine each person carries in their DNA a program segment. Now imagine the program doesn't "work" until a group of people with the right programming reach a certain location (the Ionian Nebula).
It's actually not that far off from the concept of "genetic memory."
Anyway, I would find this explanation totally acceptable.