[Antichron]

It's possible (for example, Princeton's PhD program is only four years long), but it won't be easy to do without prior graduate work. (Unless you are willing to work 25% harder than everyone else all four years.)

I plan on finishing my PhD in four years, but I don't think I'd be able to do so if I didn't already spend two years getting a Master's.

Is it desirable? It's sorta difficult to say. This year, MIT has one person who finished his PhD in four years, and he got a job offer from Stanford and flyouts from many other top departments. This is only one data point, and attributing his success to the fact that he finished in four years is likely to be pretty misleading. I imagine that if you can show that you are ambitious enough to finish your PhD in four years, it'll serve as a pretty strong signal and put you towards the top of your class, though. (At least I hope!)