Let alone Super Tuesday.
I won't include the Republicans because I'm not as well versed on their primaries but for Democrats...
In 2008, Iowa established Obama, Hillary and Edwards as the top-three candidates. Bill Richardson placed fourth and was nearly 20 points behind Edwards. There was no fourth viable candidate after Iowa and once Edwards bowed out after losing the South Carolina primary, it was a two-man(woman) race between Obama & Hillary.
In 2004, Iowa again played a role in weeding out the less viable candidates. Coming out of Iowa, Kerry and Edwards were seen as the front-runner, while Dean was on the fringe of viable. Gephardt was all but done after losing Iowa, as he had staked his whole campaign on the caucus. Dean would never recover. Joe Lieberman was pretty much done in for after coming in a distant fifth. While a few campaigns continued on through Super Tuesday, the bulk of the wins continued to go to either Kerry or Edwards.
So, I would say the logic is that we haven't had four viable candidates after the voting begins.
Even going back to 1992, you see that there isn't really four viable candidates after the voting begins.
Before the voting began, Bob Kerrey, Bill Clinton, Paul Tsongas and Jerry Brown were seen as the most viable candidates. That's four. But once the votes counted, Bob Kerrey quickly faded - only managing to win South Dakota before breaking 10% again in any primary/caucus once. It became a three-man race between Clinton, Brown and Tsongas. Tsongas would abandon his campaign on March 30th.
Unless Biden/Bernie have a major slip up and fall pretty spectacularly, there's only going to be room for one more candidate at best. It's either a woman or not. But there won't be four viable candidates a year from now.