BRUSSELS (Reuters) - EU antitrust regulators on Friday closed a four-year long investigation into Apple's ( AAPL ) rules for competing e-book and audiobook apps developers after the complainant withdrew its complaint against the iPhone maker.

"The closure of an investigation is not a finding that the conduct in question complies with EU competition rules," the European Commission, which acts as the EU antitrust enforcer, said. It did not name the complainant.

It said it would continue to monitor business practices in the European tech sector, including those of Apple ( AAPL ), both under the Digital Markets Act and competition rules.