Bain Capital agreed to buy 60 percent of WPP’s market research unit Kantar in a deal that values the business at $4 billion. The private equity firm entered into exclusive talks with ad group WPP last week after beating out rival buyout companies. WPP will retain around 60 percent of the proceeds of around $3.1 billion from the sale to cut debt to the low end of a target range, and return around $1.2 billion to shareholders, it said in a statement.