Tesla and Twitter boss Elon Musk has sold shares in the electric car manufacturer worth billions to finance the purchase of the short message service. Between Dec. 12 and 14, he sold nearly 22 million shares for a total of nearly $3.6 billion, according to a regulatory announcement released Wednesday.

It was the fourth time this year that Musk had to part with billions in Tesla shares to finance the controversial Twitter purchase.

Tesla stock has fallen by more than half this year. Investors are becoming increasingly dissatisfied with the 51-year-old's dual roles at Tesla and Twitter, especially as the electric car maker has some operational issues of its own.