The Mars departure window is limited to roughly a month by the alignment of the planets,
I have little doubt they can fly something towards Mars in that timeframe, but they will almost certainly fall short of the target,
The FAA is not used to working at such a fast pace, and it will be painful convincing them to adapt. They have already been fighting over environmental impacts on Earth even with a very slow launch rate, and Mars missions will introduce a new dimension because they could impact the environment of Mars,
We have never been so close to sending humans to Mars,
I would argue that it's less important whether SpaceX gets to Mars with crew in four to six years than that SpaceX is authentically trying to do so,
That kind of vision will continue to draw talent and capital to space that will fuel countless innovations on the way to Mars settlement, most of which will benefit primarily those of us on Earth,
I think Musk's timeline will prove a bit optimistic, but even accounting for that, we are on the threshold of a new era,
The biggest challenge will probably be getting mission launch approval from the Federal Aviation Administration (FAA),