That's the number of U.S. Senators who support marriage equality now that Bill Nelson, Heidi Heitkamp, and Joe Donnelly have endorsed equality. Nelson announced his endorsement last night, creating the Senate majority. Donnelly and Heitkamp added their voices today.
The symbolic value of having a Senate majority in favor of equality is obvious, but it could also have legislative value. Now, even if the Supreme Court somehow determines that DOMA is constitutional, a majority of the United States Senate stands ready to repeal it. Republicans always seem to find a way to sink lower, but it's hard to imagine that they'd be willing to take the political heat from filibustering something like that.
The latest announcements leave just four Democrats—Mary Landrieu, Tim Johnson, Joe Manchin, and Mark Pryor—who have yet to endorse marriage equality. They are joined by 43 Republicans in the bigot caucus.