perl -pale '$F[3]=~s/[.,]/$F[2]/g;$_=join" ",@F'
Name Position Letter String Bla 123 A ..B.,,,.C.. Bla 123 B ..a.,,,.C.. Bla 123 C ..b.,,,.A..
Name Position Letter String Bla 123 A AABAAAAACAA Bla 123 B BBaBBBBBCBB Bla 123 C CCbCCCCCACC
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