What regulatory scrutiny?
4% of Mattel's shareholders just wrote them a letter telling them to sell to Hasbro so that the 4% of shareholders can get a quick payout on their stocks. That's it. No regulatory anything is required. Mattel is going to skim the letter and politely tell these people to choke on a sea of dicks. Imagine buying a stock and then telling the company that they should just sell to a different company so your stock will be worth more, ignoring the other 90% of shareholders' opinions on that matter, or the company's profitability, or literally anything about how money or corporations work.
It's just ignorant rich people grandstanding and no one is ever going to take this seriously enough to need to look at it from a regulatory perspective. It's laughable.