Four leading Am Law 100 firms are advising on a $4 billion cash-and-stock merger announced Monday that will see Scottrade Financial Services Inc. be acquired by TD Ameritrade Holding Corp. and Toronto-Dominion Bank, better known as TD Bank.

The deal, expected to attract the scrutiny of regulators, will unite two of the largest discount brokerages in the U.S., which has seen a spate of combinations in the brokerage space so far this year. In July, Skadden, Arps, Slate, Meagher & Flom represented E*Trade Financial Corp. on its $725 million buy of online brokerage OptionsHouse from an affiliate of private equity firm General Atlantic LLC, advised by Paul, Weiss, Rifkind, Wharton & Garrison. In April, Ally Financial Inc. turned to Sullivan & Cromwell to handle its $275 million acquisition of TradeKing Group Inc.

Sullivan & Cromwell corporate partner Mark Menting, a former American Lawyer Dealmaker of the Year, is leading a team advising St. Louis-based Scottrade on its proposed sale to TD Ameritrade and TD Bank. Other lawyers from the firm working on the deal include Sullivan & Cromwell tax partner Ronald Creamer Jr., executive compensation and benefits special counsel Rebecca Coccaro, regulatory special counsel Stephen Meyer and Andrea Tokheim and associates Patrick Corrigan, Judson Finnegan, Angelyn Justian, Kristen Klein, Jameson Lloyd, Michael Portner Gartke and Regina Sorin. Scottrade’s chief legal officer is Adym Rygmyr.

Philip Richter, co-head of M&A at Fried, Frank, Harris, Shriver & Jacobson, is working with associate Roy Tannenbaum in advising The Goldman Sachs Group Inc. as financial adviser to Scottrade on the transaction, which is structured in two parts. Toronto-based TD Bank, owner of a 42 percent stake in TD Ameritrade, will invest $400 million in the Omaha-based company and pay $1.3 billion in cash to acquire Scottrade’s banking business. TD Ameritrade will then pay $2.7 billion to buy the rest of Scottrade.

Simpson Thacher & Bartlett financial institutions practice head Lee Meyerson, another former American Lawyer Dealmaker of the Year, is leading a team from the firm counseling TD Bank on its portion of the deal. Former Simpson Thacher partner Ellen Patterson, who worked closely with Meyerson over the years advising TD Bank, now serves as the Canadian financial services giant’s head of legal and compliance. Simpson Thacher and Meyerson took the lead for TD Bank in 2007 on its $8.5 billion buy of Commerce Bancorp Inc., advised by Sullivan & Cromwell, as well as TD Bank’s $6 billion acquisition of Target Corp.’s consumer credit card business in 2012. Brian Levitt, a former co-chairman of leading Canadian firm Osler, Hoskin & Harcourt, serves as chairman of the board at TD Bank.

Wachtell, Lipton, Rosen & Katz veteran financial services dealmaker Edward Herlihy and corporate partner Matthew Guest are leading a team from their firm advising TD Ameritrade on the transaction. Other Wachtell lawyers working on the matter include antitrust partner Damian Didden, bank regulatory partner Richard Kim, executive compensation and benefits partner Jeannemarie O’Brien and counsel Alicia McCarthy, finance partner Gregory Pessin, tax partner Joshua Holmes and associates Amanda Allexon, Sasha Belinkie, Kwon-Yong Jin, Emily Johnson, Jacob Kling, Minsun Lee, Bert Ma, Christina Ma, A.J. Martinez, Rachel Reisberg, Sherri Reiss, Anna Shifflet and David Sturgeon. TD Ameritrade’s general counsel is Ellen Koplow.

TD Ameritrade and TD Bank’s acquisition of Scottrade is expected to close in the second half of 2017, pending regulatory approval and other closing conditions.