[Advaita-l] upanishad mahAvAkya

[bhaskar.yr at in.abb.com]

praNAms
Hare Krishna

Kindly clarify whether shankara anywhere in prasthAna traya bhAshya says
*only four vAkya-s* from four vEda-s are the mahA vAkya-s?? If yes, please
give me the reference.  If no, then why in advaita we say only four vAkya-s
are upanishad mahAvAkyas??  is it a contribution of post shankara
vyAkhyAnakAra-s??

Hari Hari Hari Bol!!!
bhaskar