Why do you need to speak to them? If big4 was all that great, many people would be begging to come back and its not hard to come back either, there are many openings for hell but very little people signing up for it. I think it is can be quite easily concluded that all things being considered (salary, WLB, benefits) most people who left big4 are much more content outside.

Reality is big4 is just a really good stepping stone (although this benefit is disintegrating quite rapidly looking at the quality of intakes nowadays). to those who want to make partner sure by all means stay, to those who continually tell yourself that big4 is a great place for a career, you need some serious help