Human resources consulting company Watson Wyatt Worldwide Inc completed its merger with rival Towers Perrin Forster & Crosby to form a combined company called Towers Watson & Co that will make its U.S. trading debut on Monday.

Towers Watson, which competes with consultancies such as Hewitt Associates and the Mercer subsidiary of Marsh & McLennan Companies , valued the merger at $4 billion in a statement on Sunday, based on Watson Wyatt's closing stock price on December 31.

Shares of the combined company will begin trading on both the New York Stock Exchange and Nasdaq under the ticker TW on Monday. Trading of Watson Wyatt's stock under its previous ticker, WW, will be discontinued.

Towers Watson, which has about 14,000 employees, said it expects to issue about 46.9 million Class A common shares and 29.5 million Class B common shares.

The company said it would pay some former Towers Perrin shareholders who voluntarily chose to end their employment with Towers Perrin $200 million of cash and issue one-year notes worth $200 million.

Watson Wyatt Chief Executive John Haley will serve as the new company's CEO, while Towers Perrin CEO Mark Mactas will serve as president and chief operating officer.

The completion of the deal follows approval by both companies' shareholders on December 18.

Towers Watson will be headquartered in New York.

(Reporting by Phil Wahba; Editing by Steve Orlofsky)