It's hard to believe Brad Pitt and Angelina Jolie's twins - Vivienne Marcheline and Knox Leon - turned four years old on Thursday.  The two are the youngest siblings in the Pitt-Jolie brood, which also includes adopted children Maddox, 10, Pax, 8, Zahara, 7, and biological daughter Shiloh, 5.

The twins were born by Caesarian section at the Fondation Lenval hospital in Nice, France, on July 12, 2008. Vivienne was named after Angelina's mother, Marcheline Bertrand.

Shortly after they were born, Jolie talked about the difficulties of raising twins. "We're exhausted a lot, because it's twice as hard with two babies than with one," she told the German edition of Vanity Fair. "It's twice as much fun, too. When you look at them, it's like they were made for each other. They smile at each other, sniff each other, touch each other. It's just beautiful."

All of the kids are reportedly elated that their parents are finally tying the knot. Brad told The Hollywood Reporter in January, "It means so much to my kids, and they ask a lot." Their engagement was confirmed in April, and the rumor is they will wed near Provence, France. The couple has been together for seven years.