ST. LOUIS — Discount brokerage Scottrade is being acquired by rival TD Ameritrade for $4 billion.

The companies announced the deal early Monday, following several weeks of news reports about the merger. The sale is subject to regulatory approvals and is slated to close by Sept. 30, 2017. Scottrade will convert to TD Ameritrade systems in 2018.

Combined, the two companies have $944 billion in total client assets and 10 million funded client accounts.

“It gives us a lot of power to combine the two,” Scottrade’s founder and CEO Rodger Riney told the St. Louis Post-Dispatch Monday. Riney will join TD Ameritrade’s board of directors when the sale finalizes. “It is a business that has become ever more competitive as the years have gone by … and scale is more important than ever. The more clients you have, the less expensively you can offer services to clients.”

Scottrade has 3,700 employees companywide, including about 1,800 in the St. Louis region, where it’s headquartered.

Omaha, Nebraska-based TD Ameritrade says it expects to realize $450 million in combined annual expense synergies and more than $300 million in additional long-term opportunities. Savings will come in part from headcount reductions as the combined company shutters about 150 branches.

Scottrade has 1,000 investment consultants and 500 branches, but has not yet identified which locations may close.