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Nicky Hilton and David Katzenberg have ended their longterm relationship, according to New York Post's Page Six.

"They broke up quietly after four years together," a friend tells the newspaper. "The main reason was that they have been busy with their work schedules, and it has made it difficult to spend time together."

"Nicky has been back and forth traveling overseas a lot with her business. She has recently been in Asia promoting her jewelry line, and David is starting a new production company. Their schedules never seemed to match."

The insider added Hilton, 27, and, Katzenberg, 28, "remain close friends. They have been friends since they were teenagers."

Since her split with Katzenberg (who created the scripted MTV series The Hard Times of RJ Berger), Hilton has been leaning on her older sister Paris for support.

"Paris has a house in Malibu, so I've just been spending time relaxing there," Hilton told Us Weekly Wednesday at the Nylon magazine party in West Hollywood. "Paris and I are going to go to the South of France next week for some business and pleasure."