[Advaita-l] vedAnta/upanishad mahAvAkya - a query

[Bhaskar YR]

praNAms 
Hare Krishna 

Pardon my ignorance.  Whenever I hear the words 'upanishat mahAvAkya' I 
would get the doubt, what exactly is this mahAvAkya??  how can we conclude 
that  there are ONLY four mahA vAkya-s in the whole cannon of vedAnta?? 
what are all the special features that determines the  mAhAvakya-s & 
distinguishes it from other shruti vAkya-s??  when each & every word in 
shAstra denoting the same adviteeya brahman, how can we conclude that 
there is ONLY four mahAvAkya-s from all the four veda-s ??  has shankara 
specifically said anything about those four mahAvAkya-s & its superior 
efficacy in his prasthAna traya bhAshya??  Can an aspirant take initiation 
of one of these vAkya-s & repeat it like nAma japa and attain the self 
realization without undergoing traditionally prescribed shravaNa, manana 
nidhidhyAsana of vedAnta or shAstrA ??  Kindly educate me. 

Hari Hari Hari Bol!!! 
bhaskar