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4 years is a very rough estimate -- very few people actually do it in 4 years (some take as much as 7-8 years but that's really stretching it if you ask me).
At many programs, PhD students get a stipend (anywhere from $15,000 to $25,000 per year), conditional on adequate progress, for at least the first 3 years (or until they graduate, at many schools). That stipend may or may not come with some requirements (teaching, TA, RA), and those requirements may or may not come with additional pay (typically they do), and it almost always comes with a 100% tuition waiver.