The mixed Q4 results indicate that while not all financial metrics may have met expectations, the company's performance was not entirely negative. The approval of a $400 million share repurchase program is a strong signal of confidence from the company's management in its future prospects. This action is likely to be perceived positively by investors, as share buybacks often lead to an increase in the value of remaining shares. The combination of these factors contributes to the positive short-term impact on Alkermes' stock price.