A pair of New York-based office REITs are merging. On Thursday, Manhattan-based SL Green Realty Co. (SLG) agreed to buy Long Island-based Reckson Associates Realty Corp. (RA) in a cash/stock deal valued at roughly $4 billion. SLG agreed to buy RA shares for $31.68 each in cash and 0.1 shares of SLG common stock for each RA share and operating partnership unit. Once the deal is closed, RA shareholders will own roughly 15.2% of SLG. SLG won’t be paying a premium, either: The transaction values RA shares at $43.31, which is less than the $43.95 closing price on August 2.

“The acquisition of Reckson Associates provides SL Green with a huge step forward as one of the nation’s largest office REITs as we seek to continue being the sector’s outstanding performer,” says Marc Holliday, CEO of SL Green. The deal will add roughly 4 million sq. ft. of office property to SLG’s Manhattan portfolio.

“The portfolio we have acquired is a perfect fit for us — in one transaction we have added five outstanding buildings totaling over four million square feet in our core market of Manhattan, obtained key properties strategically located in the neighboring submarkets of Westchester and southern Connecticut, and expanded our highly-profitable investment platform. We were pleased to be able to work successfully with Reckson’s leadership on this and we welcome Reckson’s shareholders to the SL Green investor base.”

SLG has also agreed to sell certain RA assets to an investment group led by existing Reckson executive management and Marathon Asset Management for roughly $2.1 billion. The investor group will acquire the following from SLG:

*All of Reckson’s Long Island real estate assets

*Reckson’s 14 property Eastridge portfolio in Westchester County

*711 Westchester Avenue in White Plains, N.Y.

*Reckson’s 20 office properties and three development parcels located in New Jersey

*All of Reckson’s interests in its Australian listed property trust, plus an interest in certain structured finance instruments.

This portion of the sale is expected to close simultaneously with the closing of the acquisition of Reckson by SL Green.

“After eleven exciting years in the public arena, we have decided to recognize the significant value we have created in our portfolio and combine with SL Green to create the premiere office REIT in the metropolitan New York area,” says Scott Rechler, Chairman and Chief Executive Officer of Reckson.