Towers 3 and 4 are likely to be finished a little later than expected - the Port Authority has granted Silverstein Properties extensions of six and four months, respectively.  But here’s the good news: the extensions should encourage Merrill Lynch to call the two towers “home.”

The financial giant (its market cap is currently just over $43 billion) is being tempted to leave the World Financial Center, according to an article in The New York Sun.  And Merrill Lynch’s willingness to relocate will at least in part be determined by the layout of the new buildings.

Trading floors are rather different from cubicle-filled offices, you see, and Janno Lieber, the president of World Trade Center Properties, explained, “This extension gives us the time to design and construct a modified building foundation that could accommodate Merrill’s specialized requirements.”

Unfortunately, there’s been no word as to when Merrill Lynch might make its decision; all meetings have been held in private.

The completion dates of Towers 1 and 2 will remain completely unaffected by this development, however, and Towers 3 and 4 should be finished in 2012, regardless.