Earlier on Friday, an unverified Twitter account claiming to be Ballmer said he had taken the stake in the company.
CNBC later confirmed the report, the authenticity of Ballmer's account and his stake in Twitter.
Ballmer's 4 percent stake makes him a bigger shareholder than CEO Jack Dorsey, who owns 3.2 percent of Twitter's shares outstanding.
Ballmer was Microsoft's chief executive for over a decade and is currently the owner of the NBA's Los Angeles Clippers.
Twitter's stock intraday
—CNBC's Jessica Golden and Robert Hum contributed to this report.