A 4 party system probably should be incubated at the state level. A state could reserve a portion of state seats for the top 4 vote getting parties, say each getting 10 seats for a total of 40, which would naturally attract 4 main parties. The portion could be expanded over time leading to a split up of the current Coke vs. Pepsi duopoly we have now. Most democracies have more than 2 viable parties, because having just 2 stinks. Political interests aren't that simple.
I have a simpler solution:
1. Repeal the 17th amendment and let the States choose the senators the way they used to - that way the 30 or so red states will always outnumber the 20 or so blue states.
2. Never vote for an incumbent.