WASHINGTON—Federal regulators are investigating the case of the missing “4,” exploring the numeral’s conspicuous absence in quarterly reports that could mean companies have improperly rounded up their earnings per share to the next highest cent, according to people familiar with the matter.

Enforcement officials at the Securities and Exchange Commission have sent queries to at least 10 companies, asking the firms to provide information about accounting adjustments that could push their reported earnings per share higher, one...