NEW YORK—Massachusetts regulators are fining MassMutual $4 million and ordering it to overhaul its social-media policies after accusing the company of failing to supervise an employee whose online cheerleading of GameStop’s stock helped launch the frenzy that shook Wall Street earlier this year. The settlement announced Thursday by Secretary of the Commonwealth William Galvin centers on the actions of Keith Gill, who was an employee at a MassMutual subsidiary from April 2019 until January 2021. His tenure ended as GameStop’s stock price suddenly soared nearly 800 percent in a week, as hordes of smaller-pocketed and novice investors piled in, to the shock and awe of professionals. Gill’s job at MassMutual was to create educational materials for current and potential customers, but regulators say he was also posting more than 250 hours of videos on YouTube and sending at least 590 Tweets about investing and GameStop through accounts that were unaffiliated …