How should I state a 'PhD dropout' in my CV?
Rule #1 in resume writing is to never lie on your resume. Never, ever, ever. That doesn't mean you're doomed with a scarlet letter, of course.
I had a similar situation in the past, and worked with quite a few people to hammer out the singular line of that issue. It is quite hard to write, and you will want to talk with someone who has resume-reading/recruiting experience to make sure it sounds okay. Make sure you account for all the time you can.
Here's an example:

Mathematics, Department of Mathematics, Foo Baz University, BB, (Performed four years of studies in pursuit of PhD before exiting program) August 2011 – March 2015
B.S. in Mathematics, Department of Mathematics, John Doe University, DD, June 2011

Try to avoid using words with negative connotation, such as &quot;dropped out&quot; or &quot;failed.&quot;
Regardless of how you write it, people will ask you about it. However, it's much easier to defend your situation in an interview, especially knowing it's a question that will come up. I basically indicated that I no longer found it reasonable or practical to continue my pursuit of a medical degree and decided to focus on something that I was equally, if not better at, suited for, when I interviewed.
Since I don't know what your situation is regarding your MS, I've left that out, but if you're sure you're going to get it, you can add a line for that saying that it is tentative and to be awarded at a future point for work done during the PhD.