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For her big 4-3 on Saturday, Jennifer Aniston was feted with quite the sophisticated soiree!

An insider tells Us Weekly that the actress' hot-and-heavy beau Justin Theroux rented out a penthouse suite  overlooking West Hollywood's Sunset Strip and threw his girlfriend a party where A-list pals including Courteney Cox, Chelsea Handler, Jimmy Kimmel, Ben Stiller, Emily Blunt and Jason Bateman turned out to celebrate.

"There was minimal decor with white hurricane candles lining the room and the balcony. A bar was set up inside along with a table where guests put presents," the insider tells Us of party's atmosphere. "It was a cold night, but there were heaters on the patio. There were servers walking around with appetizers and jazz music playing."

Donning her go-to all-black garb, Aniston worked a sleeveless black dress and sipped on martinis while she happily made her way around the room to spend time with each of her guests.

"She seemed very excited to see her friends, but she never got too far from Justin," the insider tells Us. "Every once in awhile, Justin would come up from behind and give her a big hug and kiss. They would embrace and whisper to each other for a minute but then go back to socializing. She seemed very relaxed."

"The party went late night and many people stayed past midnight," the insider adds. "Jen and Justin were some of the last people to leave."