WASHINGTON—The U.S. Justice Department and Federal Trade Commission said Monday that they had been notified of more than 4,000 proposed transactions between March 2020 and September 2021, sharply higher than normal. The agencies, which share the job of enforcing antitrust law, created an online portal to accept notifications of proposed transactions in March 2020, when the government and companies started closing offices to slow the spread of the coronavirus pandemic. Between then and Sept. 10, 2021, roughly 18 months, the portal took in filings related to more than 4,000 transaction, the agencies said in a report. That compares with 1,637 for the 2020 fiscal year, 2,089 for fiscal 2019, and 2,111 for the 2018 fiscal year, the report said. A tally that the FTC has on its website shows that filings started picking up in the fall of 2020, with 424 in November 2020 alone. There have been more than …