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.