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How Do I Build on Small Firm Experience to Get Considered for a Position with a Big 4 Firm?

Another career conundrum from the mailbag:
I am just interested in some quick advice and I greatly appreciate your advice ahead of time.
I graduated a year ago with my degree in accounting from a fairly good accounting program. Unfortunately, my GPA was only about a 3.23. My major GPA was a little bit higher than that but not much. I had troubles freshman year with extreme food allergies and was never able to really overcome that. 
I have a year of experience from a very small CPA firm doing a ton of tax returns, audits and some bookkeeping and compilations. I left working there because of the work environment and my manager was looking for someone to be "100% committed to the organization". My manager had an open door policy and found out I was looking for a new job and didn't look too kindly on it. 
I have passed two parts of the CPA exam and am studying full time to pass the next two parts by November 1. 
Do you have any advice for me to be considered at the Big 4 accounting firms? 
Could I work another 2-3 years at smaller, regional firms and then move into a Big 4 firm or would it be better to look at a masters in Tax to be able to prove my diligence and getting a higher GPA? I have been networking with as many recruiters as possible.