I've worked in the Big 4 consulting game for a over 4 years now and I wanted to shed some light on their culture. Disclaimer: my interpretation of PWC and Deloitte is only based on anecdotal feedback from colleagues/ friends who are working there, and my interactions at bidder meetings.
Caveat: this perspective applies to Asia-Pacific and the United States

EY: Dyslexics untie, Alcoholics Unite, High school 2.0

EY is essentially an amalgamation of alcoholics who seem to stay sober during work hours, but let loose at any time they can. I've worked in both industry and other consulting roles, but I had the most fun at EY. Can't recall the number of times I would rock up to a party/ Friday night drinks and see a partner, who would be a down-right straight-shooter during business hours, let loose buying shots for everyone and dancing with his shirt undone. Although EY is full of extremely smart people, there were so many instances I could recall in which juniors would get away with making stupid errors in front of clients, but because of the relationship no one really gave a shit.

KPMG: Jack of All Trades, Master of none

KPMG dabbed in most forms of advisory/ consulting service, but it never really went extremely deep in any particular area apart from audit and bread+butter FDD. Some KPMG offices were essentially body shops and this will be the reason why it will never grow and improve it's brand reputation in the long-term. KPMG's culture has much to be desired. It's essentially as if you were reliving your group assignment days at University, when everyone would work individually but come together only when they need to. This is the result of KPMG's perpetual acquisition pipeline. That is, because KPMG is forever acquiring new businesses, there is continuous change and people tend to physically sit and stick with their superiors who will buddy them up at the next roundtable. This behaviour is quite evident in most divisions and it's a real let down in all honesty.

PWC: I think, therefore I am

PWC is continually number 1 among the big 4, although their strengths DO vary significantly from geography to geography e.g. PWC is heavy banking focused in Australia, heavy Financial Services in London. PWCers pride themselves on their firm's flexible working arrangements by expressing their undying love of their firm's policies on linkedIn. PWC partners are forever balding and seem to be the most arrogant out of all of the big 4 (based on my interactions with them in deal meetings), their implicit authority is almost unpalatable, but felt by most PWCers from Manager down. PWC seem to pigeon hole their staff very early on in their career, and once pigeonholed in a particular sector it's hard to break out.

Deloitte: Toxic culture, self-serving leadership

This statement is an overall summary of the feedback i've received from my network. Whether people had / are having good/ bad experiences, they all seem to say the same thing: it's extremely toxic.
I've heard some shocking stories about clicky circles and self-serving partners/directors and individuals being asked to leave or managed out. Deloitte also appear to dabble in anything they can possibly do without any true depth. So I'd highly recommend you to do your research on Deloitte as it applies to your geography and validate any of the above.

Thought I'd share some insight / comic relief so don't grill me too hard because of your deep-rooted firm faction.