PARIS (Reuters) - Advertising group Publicis (>> PUBLICIS GROUPE) pledged on Thursday to grow revenue faster than rivals and expand margins in a four-year plan building on its $3.7 billion acquisition of digital specialist Sapient (>> Sapient Corporation).

Helped by Sapient's strength in e-commerce and technology services, the French group said it would target organic revenue growth 2 percentage points above the industry average, starting in 2016. It also aims to raise the operating margin by 2 to 4 percentage points from its 2012 pro-forma level of 15.3 percent, with Sapient taken into account.

Publicis Chief Executive Maurice Levy is trying to win back investor support after a tough year in which a mega-merger with U.S. peer Omnicom (>> Omnicom Group Inc.) fell apart and the company's growth lagged behind competitors.

Publicis shares are down 11 percent this year, while larger rival WPP (>> WPP PLC) is about 2 percent lower and U.S.-based Omnicom (>> Omnicom Group Inc.) and Interpublic (>> Interpublic Group of Companies Inc) are up 5 and 16 percent respectively.

The four-year plan unveiled on Thursday would increase shareholder returns, raising the dividend payout ratio from 35 percent in 2015 to 42 percent in 2018. The early repayment of convertible bonds will also return 700-800 million euros (549-628 million pounds) to investors, the company said.

(Reporting by Leila Abboud and Gwenaelle Barzic; Editing by Laurence Frost)