Welcome to the forum. Regarding your questions, the three routes you describe are very different, so first you need to decide what you want out of your career. Staying at a Big 4 firm is great preparation to move into a senior accounting role (controller/asst. controller). CFO it's a little less certain. A few partners I worked for at my Big 4 firm left to become CFOs, but they were pretty big shot partners to begin with. I think a lot of companies want their CFO to have more finance experience as well as being smart about accounting.
A PhD in accounting is really only worthwhile if you want to go into academics and teaching. Nothing wrong with that career path, but it's a very different path than controller or CFO.
An MBA gives you the opportunity to broaden your business expertise and would allow you to go into fields that a CPA would not. I know CPAs who got their MBA and now work in banking, so that's definitely an option. I personally am going from being a CPA into doing marketing. Because an MBA is a very unspecialized degree, there's really few limits on what areas of business you can go into with an MBA, which is not the case for a CPA.
As for whether Big 4 experience is something business schools look for, the answer is not so straightforward. On the one hand, being hired by a Big 4 firm tells the adcom something about your skills and accomplishments, because of the hiring standards that the firms have. However, because there are so many Big 4 accountants, you still need to have something on your resume (preferably multiple somethings) to make you stand out from the crowd.
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