Why the mk4? Because the 1-series body has been the staple of photojournalism since before there was a D after the 1. Its the best Canon makes and for people like me who sometimes have to live out of their cars while on assignment, have their cameras on them all the time with no bags and no lens caps they are the only cameras that can take the abuse. Also the mk4 is popular because most of us aren't rich hobbyists who buy 1dx's and 5d3's when they come out. It always makes me laugh when I come on here and see people talking about how the new camera will help them shoot better. Most of us shoot what the companies give us, or our personal gear is a reflection of 'what will make me the most money,' not 'what will cost me the most money.'
And there it is - ^^ - the definitive answer! 
.DAVID.






