why do you assume the fourth is a 'her'?
can i call wildcat meowie?

love the way his mind works.
but i digress. just personally as an identical twin (how many pairs of twins are there here?

):
it's a nature vs nurture argument. To put everything down to DNA and say that just because twins share the same genome, they'd be exactly the same, is too absolute perhaps?
because who we are is ultimately shaped by the environment. and even though the
macro environment may be the same (eg, same socio-cultural background, same family and place of school/growing up etc), the
daily micro-environment will be different (the individualised experiences one has).
that would shape personality, wouldn't it?
aelan pointed out rightly too: most DNA as far as humans are concerned (elf majored in microbiology, btw) comes from the main nucleus of the cell itself. but in the cytoplasm resides the mitochondrial DNA (which comes from the maternal line), and so, not 100% of the DNA in twins are the same. which accounts for the differences in dental records and thumbprints, even though the main DNA profile (blood type etc) are the same.
i'm visual. so to simplify things:
imagine two snowflakes of almost the same composition in the sky. then they fall. along the way, they'd meet different obstacles and winds and all that. that's what shapes them differently, the wings of the crystals will be different, even though the core would (likely) be mostly the same.
i hope i managed some clarity in the above rambling.