To be honest the big 4 companies are over rated for training purposes. I just got my FAE's last week and I started with a medium sized company. You get more exposure in a medium sized company than you do with the bigger firms. In the big 4 you are designated a specific industry and you may not ever get out of it whereas medium sized you will work on a range of different clients. i have spoken to a lot of people that have qualified with big 4 and as soon as there contracts finish they leave to smaller firms to get more experience. I was also talking to one of my managers last year and her friend in PWC was asking if any people in my firm were finishing their contacts soon because they wanted people with more experience and training than what they give their trainees.
Just something to think about if they don't go to plan there are plenty of smaller firms that you will get great experience with and then after your contract finishes then move to the big 4.