Will macbook pro mid 2012 gt650m show 2560 resolution on a 4k display?
No it will not. According to http://support.apple.com/en-us/HT6008:

"You can use 4K displays and Ultra HD TVs with these Mac computers.

MacBook Pro (Retina, Late 2013 and later)
Mac Pro (Late 2013)
iMac (27-inch, Late 2013 and later)
Mac mini (Late 2014)"