Four states and the District of Columbia have already legalized weed and plans are already afoot in several more for this year or next, either by voter initiative (where that option is available) or through the legislature. But which states will be next, and how fast will the dominoes fall? After all, we're unlikely to see the end of federal pot prohibition until a lot more than four states have legalized it, so how long is it going to take to reach that critical mass? (See also: 7 states that are next in line to legalize marijuana)