A several-way debate might help.
But the debates are only open to candidates who poll above a certain percentage of the electorate (I believe it is 15%, but am not certain). So you need to get two third party/independent tickets running at 15% or more of the vote, preferably without the Democrats or republicans completly falling apart.
Now, if you want to be technical, the United States has already had at least two real four party elections (or four person) elections: 1872 and 1824. Neither is really quite what the scenario seems to envision, however. 1824 saw Clay, Crawford, and Adams all basically camiagning to stop Andrew Jackson, and as a result forced the election to go to the House of Representatives (adams won). 1872 was really an abberation. Grant never faced a serious challenge, and his opponent, Horace Greeley, died before the Electoral college could meet. So, in addition to Grant and Greeley (who still got electoral votes), four different people recieved electoral votes from Greeley supporters.
But I dont think that, especially in the modern day, four party presidential elections are viable without the electoral college being removed. Other parties would be reduced to occasional moments of glory (Perot '92), or being a spoiler (Nader '00, what Barr was expected to do in '08), and occasionally pushing the election to the House. Now, if you had alternative parties holding any strength in Congress, this might become possible, as they become the balance of power, but this would be difficult to maintain.