I agree with what both guys have said.
In my experience, at mid-tier we received a textbook allowance of $50 per subject, and 2 days of leave per subject, per exam (Ie. the day before and the day of the exam).
I didn't see anything regarding training for CA students in my time, though they did receive a subsidy for books and course fees.
We had regular staff training, but there wasn't like, a specific time where somebody sat down with all the CA students and coached them.
I also think another advantage of Big 4 is the opportunity to try more branches of accounting rather than one job. I know when I left my job there wasn't an opportunity to just move to say, Audit instead. I've heard a few people mention some Big 4 let you try out different fields before settling down, and I presume there would be more room to move to a different department purely because of the size of the firms.
I think Seremify has mentioned in the past big 4 like PwC may give more leeway to do an exchange overseas while your in uni. I know for most smaller firms loosing a member of staff for 6 months to a year wouldn't be an option.
I would also say Big 4 would be good for your uni studies/networking/social life because there are more people your age than a smaller firm. At my old firm there was 13 cadets, and at my new job I'm the youngest person by 4 years, and then the next youngest person is 35 or something
I know there are other young people in our city office, but I don't see them often
It was really great being able to talk to other people going through the same process as you about work and studies and everything. I made some really great friendships at my old job.